Lithium-ion Battery Technology
Jan. 01, 1970
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A lithium-ion battery, or Li-ion battery, is a type of rechargeable battery that uses the reversible intercalation of Li+ ions into electronically conducting solids to store energy. Li-ion batteries are characterized by higher specific energy, energy density, and energy efficiency and a longer cycle life and calendar life than other types of rechargeable batteries. Also noteworthy is a dramatic improvement in lithium-ion battery properties after their market introduction in ; over the following 30 years, their volumetric energy density increased threefold while their cost dropped tenfold. In late global demand passed 1 terawatt-hour per year, while production capacity was more than twice that.
The invention and commercialization of Li-ion batteries has had a large impact on technology, as recognized by the Nobel Prize in Chemistry. Li-ion batteries have enabled portable consumer electronics, laptop computers, cellular phones, and electric cars. Li-ion batteries also see significant use for grid-scale energy storage as well as military and aerospace applications.
M. Stanley Whittingham conceived intercalation electrodes in the s and created the first rechargeable lithium-ion battery, based on a titanium disulfide cathode and a lithium-aluminium anode, although it suffered from safety problems and was never commercialized. John Goodenough expanded on this work in by using lithium cobalt oxide as a cathode. The first prototype of the modern Li-ion battery, which uses a carbonaceous anode rather than lithium metal, was developed by Akira Yoshino in and commercialized by a Sony and Asahi Kasei team led by Yoshio Nishi in . Whittingham, Goodenough, and Yoshino were awarded the Nobel Prize in Chemistry for their contributions to the development of lithium-ion batteries.
Lithium-ion batteries can be a fire or explosion hazard as they contain flammable electrolytes. Progress has been made in the development and manufacturing of safer lithium-ion batteries. Lithium-ion solid-state batteries are being developed to eliminate the flammable electrolyte. Recycled batteries can create toxic waste, including from toxic metals, and are a fire risk.[citation needed] Both lithium and other minerals can have significant issues in mining, with lithium being water intensive in often arid regions and other minerals used in some Li-ion chemistries potentially being conflict minerals such as cobalt. Environmental issues have encouraged some researchers to improve mineral efficiency and find alternatives such as lithium iron phosphate lithium-ion chemistries or non-lithium-based battery chemistries such as sodium-ion and iron-air batteries.
"Li-ion battery" can be considered a generic term involving at least 12 different chemistries; see List of battery types. Lithium-ion cells can be manufactured to optimize energy density or power density. Handheld electronics mostly use lithium polymer batteries (with a polymer gel as an electrolyte), a lithium cobalt oxide (LiCoO
2) cathode material, and a graphite anode, which together offer high energy density. Lithium iron phosphate (LiFePO
4), lithium manganese oxide (LiMn
2O
4 spinel, or Li
2MnO
3-based lithium-rich layered materials, LMR-NMC), and lithium nickel manganese cobalt oxide (LiNiMnCoO
2 or NMC) may offer longer life and a higher discharge rate. NMC and its derivatives are widely used in the electrification of transport, one of the main technologies (combined with renewable energy) for reducing greenhouse gas emissions from vehicles.
The growing demand for safer, more energy-dense, and longer-lasting batteries is driving innovation beyond conventional lithium-ion chemistries. According to a market analysis report by Consegic Business Intelligence, next-generation battery technologies—including lithium-sulfur, solid-state, and lithium-metal variants are projected to see significant commercial adoption due to improvements in performance and increasing investment in R&D worldwide. These advancements aim to overcome limitations of traditional lithium-ion systems in areas such as electric vehicles, consumer electronics, and grid storage.
Research on rechargeable Li-ion batteries dates to the s; one of the earliest examples is a CuF
2/Li battery developed by NASA in . The breakthrough that produced the earliest form of the modern Li-ion battery was made by British chemist M. Stanley Whittingham in , who first used titanium disulfide (TiS
2) as a cathode material, which has a layered structure that can take in lithium ions without significant changes to its crystal structure. Exxon tried to commercialize this battery in the late s, but found the synthesis expensive and complex, as TiS
2 is sensitive to moisture and releases toxic hydrogen sulfide (H
2S) gas on contact with water. More prohibitively, the batteries were also prone to spontaneously catch fire due to the presence of metallic lithium in the cells. For this, and other reasons, Exxon discontinued the development of Whittingham\'s lithium-titanium disulfide battery.
In , working in separate groups Ned A. Godshall et al., and, shortly thereafter, Koichi Mizushima and John B. Goodenough, after testing a range of alternative materials, replaced TiS
2 with lithium cobalt oxide (LiCoO
2, or LCO), which has a similar layered structure but offers a higher voltage and is much more stable in air. This material would later be used in the first commercial Li-ion battery, although it did not, on its own, resolve the persistent issue of flammability.
These early attempts to develop rechargeable Li-ion batteries used lithium metal anodes, which were ultimately abandoned due to safety concerns, as lithium metal is unstable and prone to dendrite formation, which can cause short-circuiting. The eventual solution was to use an intercalation anode, similar to that used for the cathode, which prevents the formation of lithium metal during battery charging. The first to demonstrate lithium ion reversible intercalation into graphite anodes was Jürgen Otto Besenhard in . Besenhard used organic solvents such as carbonates, however these solvents decomposed rapidly providing short battery cycle life. Later, in , Rachid Yazami used a solid organic electrolyte, polyethylene oxide, which was more stable.
In , Akira Yoshino at Asahi Kasei Corporation discovered that petroleum coke, a less graphitized form of carbon, can reversibly intercalate Li-ions at a low potential of ~0.5 V relative to Li+ /Li without structural degradation. Its structural stability originates from its amorphous carbon regions, which serving as covalent joints to pin the layers together. Although it has a lower capacity compared to graphite (~Li0.5C6, 186 mAh g–1), it became the first commercial intercalation anode for Li-ion batteries owing to its cycling stability. In , Yoshino patented what would become the first commercial lithium-ion battery using this anode. He used Goodenough\'s previously reported LiCoO2 as the cathode and a carbonate ester-based electrolyte. The battery was assembled in the discharged state, which made it safer and cheaper to manufacture. In , using Yoshino\'s design, Sony began producing and selling the world\'s first rechargeable lithium-ion batteries. The following year, a joint venture between Toshiba and Asahi Kasei Co. also released a lithium-ion battery.
Significant improvements in energy density were achieved in the s by replacing Yoshino\'s soft carbon anode first with hard carbon and later with graphite. In , Jeff Dahn and two colleagues at Dalhousie University (Canada) reported reversible intercalation of lithium ions into graphite in the presence of ethylene carbonate solvent (which is solid at room temperature and is mixed with other solvents to make a liquid). This represented the final innovation of the era that created the basic design of the modern lithium-ion battery.
In , global lithium-ion battery production capacity was 20 gigawatt-hours. By , it was 28 GWh, with 16.4 GWh in China. Global production capacity was 767 GWh in , with China accounting for 75%. Production in is estimated by various sources to be between 200 and 600 GWh, and predictions for range from 400 to 1,100 GWh.
In , John B. Goodenough, Rachid Yazami and Akira Yoshino received the IEEE Medal for Environmental and Safety Technologies for developing the lithium-ion battery; Goodenough, Whittingham, and Yoshino were awarded the Nobel Prize in Chemistry "for the development of lithium-ion batteries". Jeff Dahn received the ECS Battery Division Technology Award () and the Yeager award from the International Battery Materials Association ().
In April , CATL announced that it would begin scaled-up production of its semi-solid condensed matter battery that produces a then record 500 Wh/kg. They use electrodes made from a gelled material, requiring fewer binding agents. This in turn shortens the manufacturing cycle. One potential application is in battery-powered airplanes. Another new development of lithium-ion batteries are flow batteries with redox-targeted solids that use no binders or electron-conducting additives, and allow for completely independent scaling of energy and power.
Generally, the negative electrode of a conventional lithium-ion cell is made from graphite. The positive electrode is typically a metal oxide or phosphate. The electrolyte is a lithium salt in an organic solvent. The negative electrode (which is the anode when the cell is discharging) and the positive electrode (which is the cathode when discharging) are prevented from shorting by a separator. The electrodes are connected to the powered circuit through two pieces of metal called current collectors.
The negative and positive electrodes swap their electrochemical roles (anode and cathode) when the cell is charged. Despite this, in discussions of battery design the negative electrode of a rechargeable cell is often just called "the anode" and the positive electrode "the cathode".
In its fully lithiated state of LiC6, graphite correlates to a theoretical capacity of coulombs per gram (372 mAh/g). The positive electrode is generally one of three materials: a layered oxide (such as lithium cobalt oxide), a polyanion (such as lithium iron phosphate) or a spinel (such as lithium manganese oxide). More experimental materials include graphene-containing electrodes, although these remain far from commercially viable due to their high cost.
Lithium reacts vigorously with water to form lithium hydroxide (LiOH) and hydrogen gas. Thus, a non-aqueous electrolyte is typically used, and a sealed container rigidly excludes moisture from the battery pack. The non-aqueous electrolyte is typically a mixture of organic carbonates such as ethylene carbonate and propylene carbonate containing complexes of lithium ions. Ethylene carbonate is essential for making solid electrolyte interphase on the carbon anode, but since it is solid at room temperature, a liquid solvent (such as propylene carbonate or diethyl carbonate) is added.
The electrolyte salt is almost always[citation needed] lithium hexafluorophosphate (LiPF
6), which combines good ionic conductivity with chemical and electrochemical stability. The hexafluorophosphate anion is essential for passivating the aluminium current collector used for the positive electrode. A titanium tab is ultrasonically welded to the aluminium current collector.
Other salts like lithium perchlorate (LiClO
4), lithium tetrafluoroborate (LiBF
4), and lithium bis(trifluoromethanesulfonyl)imide (LiC
2F
6NO
4S
2) are frequently used in research in tab-less coin cells, but are not usable in larger format cells, often because they are not compatible with the aluminium current collector. Copper (with a spot-welded nickel tab) is used as the current collector at the negative electrode.
Current collector design and surface treatments may take various forms: foil, mesh, foam (dealloyed), etched (wholly or selectively), and coated (with various materials) to improve electrical characteristics.
Depending on materials choices, the voltage, energy density, life, and safety of a lithium-ion cell can change dramatically. Current effort has been exploring the use of novel architectures using nanotechnology to improve performance. Areas of interest include nano-scale electrode materials and alternative electrode structures.
The reactants in the electrochemical reactions in a lithium-ion cell are the materials of the electrodes, both of which are compounds containing lithium atoms. Although many thousands of different materials have been investigated for use in lithium-ion batteries, only a very small number are commercially usable. All commercial Li-ion cells use intercalation compounds as active materials. The negative electrode is usually graphite, although silicon is often mixed in to increase the capacity. The electrolyte is usually lithium hexafluorophosphate, dissolved in a mixture of organic carbonates. A number of different materials are used for the positive electrode, such as LiCoO2, LiFePO4, and lithium nickel manganese cobalt oxides.
During cell discharge the negative electrode is the anode and the positive electrode the cathode: electrons flow from the anode to the cathode through the external circuit. An oxidation half-reaction at the anode produces positively charged lithium ions and negatively charged electrons. The oxidation half-reaction may also produce uncharged material that remains at the anode. Lithium ions move through the electrolyte; electrons move through the external circuit toward the cathode where they recombine with the cathode material in a reduction half-reaction. The electrolyte provides a conductive medium for lithium ions but does not partake in the electrochemical reaction. The reactions during discharge lower the chemical potential of the cell, so discharging transfers energy from the cell to wherever the electric current dissipates its energy, mostly in the external circuit.
During charging these reactions and transports go in the opposite direction: electrons move from the positive electrode to the negative electrode through the external circuit. To charge the cell the external circuit has to provide electrical energy. This energy is then stored as chemical energy in the cell (with some loss, e. g., due to coulombic efficiency lower than 1).
Both electrodes allow lithium ions to move in and out of their structures with a process called insertion (intercalation) or extraction (deintercalation), respectively.
As the lithium ions "rock" back and forth between the two electrodes, these batteries are also known as "rocking-chair batteries" or "swing batteries" (a term given by some European industries).
The following equations exemplify the chemistry (left to right: discharging, right to left: charging).
The negative electrode half-reaction for the graphite is
The positive electrode half-reaction in the lithium-doped cobalt oxide substrate is
The full reaction being
The overall reaction has its limits. Overdischarging supersaturates lithium cobalt oxide, leading to the production of lithium oxide, possibly by the following irreversible reaction:
Overcharging up to 5.2 volts leads to the synthesis of cobalt (IV) oxide, as evidenced by x-ray diffraction:
The cell\'s energy is equal to the voltage times the charge. Each gram of lithium represents Faraday\'s constant/6.941, or 13,901 coulombs. At 3 V, this gives 41.7 kJ per gram of lithium, or 11.6 kWh per kilogram of lithium. This is slightly more than the heat of combustion of gasoline; however, lithium-ion batteries as a whole are still significantly heavier per unit of energy due to the additional materials used in production.
Note that the cell voltages involved in these reactions are larger than the potential at which an aqueous solutions would electrolyze.
During discharge, lithium ions (Li+
) carry the current within the battery cell from the negative to the positive electrode, through the non-aqueous electrolyte and separator diaphragm.
During charging, an external electrical power source applies an over-voltage (a voltage greater than the cell\'s own voltage) to the cell, forcing electrons to flow from the positive to the negative electrode. The lithium ions also migrate (through the electrolyte) from the positive to the negative electrode where they become embedded in the porous electrode material in a process known as intercalation.
Energy losses arising from electrical contact resistance at interfaces between electrode layers and at contacts with current collectors can be as high as 20% of the entire energy flow of batteries under typical operating conditions.
The charging procedures for single Li-ion cells, and complete Li-ion batteries, are slightly different:
During the constant current phase, the charger applies a constant current to the battery at a steadily increasing voltage, until the top-of-charge voltage limit per cell is reached.
During the balance phase, the charger/battery reduces the charging current (or cycles the charging on and off to reduce the average current) while the state of charge of individual cells is brought to the same level by a balancing circuit until the battery is balanced. Balancing typically occurs whenever one or more cells reach their top-of-charge voltage before the other(s), as it is generally inaccurate to do so at other stages of the charge cycle. This is most commonly done by passive balancing, which dissipates excess charge as heat via resistors connected momentarily across the cells to be balanced. Active balancing is less common, more expensive, but more efficient, returning excess energy to other cells (or the entire pack) via a DC-DC converter or other circuitry. Balancing most often occurs during the constant voltage stage of charging, switching between charge modes until complete. The pack is usually fully charged only when balancing is complete, as even a single cell group lower in charge than the rest will limit the entire battery\'s usable capacity to that of its own. Balancing can last hours or even days, depending on the magnitude of the imbalance in the battery.
During the constant voltage phase, the charger applies a voltage equal to the maximum cell voltage times the number of cells in series to the battery, as the current gradually declines towards 0, until the current is below a set threshold of about 3% of initial constant charge current.
Periodic topping charge about once per 500 hours. Top charging is recommended to be initiated when voltage goes below 4.05 V/cell.
Failure to follow current and voltage limitations can result in an explosion.
Charging temperature limits for Li-ion are stricter than the operating limits. Lithium-ion chemistry performs well at elevated temperatures but prolonged exposure to heat reduces battery life. Li‑ion batteries offer good charging performance at cooler temperatures and may even allow "fast-charging" within a temperature range of 5 to 45 °C (41 to 113 °F). Charging should be performed within this temperature range. At temperatures from 0 to 5 °C charging is possible, but the charge current should be reduced. During a low-temperature (under 0 °C) charge, the slight temperature rise above ambient due to the internal cell resistance is beneficial. High temperatures during charging may lead to battery degradation and charging at temperatures above 45 °C will degrade battery performance, whereas at lower temperatures the internal resistance of the battery may increase, resulting in slower charging and thus longer charging times.[better source needed]
Batteries gradually self-discharge even if not connected and delivering current. Li-ion rechargeable batteries have a self-discharge rate typically stated by manufacturers to be 1.5–2% per month.
The rate increases with temperature and state of charge. A study found that for most cycling conditions self-discharge was primarily time-dependent; however, after several months of stand on open circuit or float charge, state-of-charge dependent losses became significant. The self-discharge rate did not increase monotonically with state-of-charge, but dropped somewhat at intermediate states of charge. Self-discharge rates may increase as batteries age. In , self-discharge per month was measured at 8% at 21 °C, 15% at 40 °C, 31% at 60 °C. By , monthly self-discharge rate was estimated at 2% to 3%, and 2–3% by .
By comparison, the self-discharge rate for NiMH batteries dropped, as of , from up to 30% per month for previously common cells to about 0.08–0.33% per month for low self-discharge NiMH batteries, and is about 10% per month in NiCd batteries.[citation needed]
Transition metal oxides (TMOs) are widely used as cathode materials in lithium-ion batteries as the variable oxidation state of transition metal cations allows oxides of these metals to reversibly host lithium ions (Li⁺) and undergo efficient redox (reduction-oxidation) reactions. Their layered or framework structures allow Li⁺ insertion/extraction during charging/discharging, while their transition metals and oxygen anions participate in electron transfer, enabling high energy density and stability. Three classes of cathode materials in lithium-ion batteries have been commercialized: (1) layered oxides, (2) spinel oxides and (3) oxoanion complexes. All of them were discovered by John Goodenough and his collaborators.
LiCoO2 was used in the first commercial lithium-ion battery made by Sony in . The layered oxides have a pseudo-tetrahedral structure comprising layers made of MO6 octahedra separated by interlayer spaces that allow for two-dimensional lithium-ion diffusion.[citation needed] The band structure of LixCoO2 allows for true electronic (rather than polaronic) conductivity. However, due to an overlap between the Co4+ t2g d-band with the O2- 2p-band, the x must be >0.5, otherwise O2 evolution occurs. This limits the charge capacity of this material to ~140 mA h g−1.
Several other first-row (3d) transition metals also form layered LiMO2 salts. Some can be directly prepared from lithium oxide and M2O3 (e.g. for M=Ti, V, Cr, Co, Ni), while others (M= Mn or Fe) can be prepared by ion exchange from NaMO2. LiVO2, LiMnO2 and LiFeO2 suffer from structural instabilities (including mixing between M and Li sites) due to a low energy difference between octahedral and tetrahedral environments for the metal ion M. For this reason, they are not used in lithium-ion batteries. However, Na+ and Fe3+ have sufficiently different sizes that NaFeO2 can be used in sodium-ion batteries.
Similarly, LiCrO2 shows reversible lithium (de)intercalation around 3.2 V with 170–270 mAh/g. However, its cycle life is short, because of disproportionation of Cr4+ followed by translocation of Cr6+ into tetrahedral sites. On the other hand, NaCrO2 shows a much better cycling stability. LiTiO2 shows Li+ (de)intercalation at a voltage of ~1.5 V, which is too low for a cathode material.
These problems leave LiCoO
2 and LiNiO
2 as the only practical layered oxide materials for lithium-ion battery cathodes. The cobalt-based cathodes show high theoretical specific (per-mass) charge capacity, high volumetric capacity, low self-discharge, high discharge voltage, and good cycling performance. Unfortunately, they suffer from a high cost of the material. For this reason, the current trend among lithium-ion battery manufacturers is to switch to cathodes with higher Ni content and lower Co content.
In addition to a lower (than cobalt) cost, nickel-oxide based materials benefit from the two-electron redox chemistry of Ni: in layered oxides comprising nickel (such as nickel-cobalt-manganese NCM and nickel-cobalt-aluminium oxides NCA), Ni cycles between the oxidation states +2 and +4 (in one step between +3.5 and +4.3 V), cobalt- between +2 and +3, while Mn (usually >20%) and Al (typically, only 5% is needed) remain in +4 and 3+, respectively. Thus increasing the Ni content increases the cyclable charge. For example, NCM111 shows 160 mAh/g, while LiNi0.8Co0.1Mn0.1O2 (NCM811) and LiNi0.8Co0.15Al0.05O2 (NCA) deliver a higher capacity of ~200 mAh/g. NCM and NCA batteries are collectively called Ternary Lithium Batteries.
It is worth mentioning so-called "lithium-rich" cathodes that can be produced from traditional NCM ( LiMO2, where M=Ni, Co, Mn) layered cathode materials upon cycling them to voltages/charges corresponding to Li:M<0.5. Under such conditions a new semi-reversible redox transition at a higher voltage with ca. 0.4-0.8 electrons/metal site charge appears. This transition involves non-binding electron orbitals centered mostly on O atoms. Despite significant initial interest, this phenomenon did not result in marketable products because of the fast structural degradation (O2 evolution and lattice rearrangements) of such "lithium-rich" phases.
LiMn2O4 adopts a cubic lattice, which allows for three-dimensional lithium-ion diffusion. Manganese cathodes are attractive because manganese is less expensive than cobalt or nickel. The operating voltage of Li-LiMn2O4 battery is 4 V, and ca. one lithium per two Mn ions can be reversibly extracted from the tetrahedral sites, resulting in a practical capacity of <130 mA h g–1. However, Mn3+ is not a stable oxidation state, as it tends to disporportionate into insoluble Mn4+ and soluble Mn2+. LiMn2O4 can also intercalate more than 0.5 Li per Mn at a lower voltage around +3.0 V. However, this results in an irreversible phase transition due to Jahn-Teller distortion in Mn3+:t2g3eg1, as well as disproportionation and dissolution of Mn3+.
An important improvement of Mn spinel are related cubic structures of the LiMn1.5Ni0.5O4 type, where Mn exists as Mn4+ and Ni cycles reversibly between the oxidation states +2 and +4. This materials show a reversible Li-ion capacity of ca. 135 mAh/g around 4.7 V. Although such high voltage is beneficial for increasing the specific energy of batteries, the adoption of such materials is currently hindered by the lack of suitable high-voltage electrolytes. In general, materials with a high nickel content are favored in , because of the possibility of a 2-electron cycling of Ni between the oxidation states +2 and +4.
LiV2O4 (lithium vanadium oxide) operates as a lower (ca. +3.0 V) voltage than LiMn2O4, suffers from similar durability issues, is more expensive, and thus is not considered of practical interest.
Around Manthiram discovered that oxoanions (molybdates and tungstates in that particular case) cause a substantial positive shift in the redox potential of the metal-ion compared to oxides. In addition, these oxoanionic cathode materials offer better stability/safety than the corresponding oxides. However, they also suffer from poor electronic conductivity due to the long distance between redox-active metal centers, which slows down the electron transport. This necessitates the use of small (less than 200 nm) cathode particles and coating each particle with a layer of electronically-conducting carbon. This reduces the packing density of these materials.
Although numerous combinations of oxoanions (sulfate, phosphate, silicate) with various metals (mostly Mn, Fe, Co, Ni) have been studied, LiFePO4 is the only one that has been commercialized. Although it was originally used primarily for stationary energy storage due to its lower energy density compared to layered oxides, it has begun to be widely used in electric vehicles since the s.
Positive electrode Technology Major producers () Target application Advantages Lithium nickel manganese cobalt oxideNegative electrode materials are traditionally constructed from graphite and other carbon materials, although newer silicon-based materials are being increasingly used (see Nanowire battery). In , 89% of lithium-ion batteries contained graphite (43% artificial and 46% natural), 7% contained amorphous carbon (either soft carbon or hard carbon), 2% contained lithium titanate (LTO) and 2% contained silicon or tin-based materials.
These materials are used because they are abundant, electrically conducting and can intercalate lithium ions to store electrical charge with modest volume expansion (~10%). Graphite is the dominant material because of its low intercalation voltage and excellent performance. Various alternative materials with higher capacities have been proposed, but they usually have higher voltages, which reduces energy density. Low voltage is the key requirement for anodes; otherwise, the excess capacity is useless in terms of energy density.
Negative electrode Technology Energy density Durability Company Target application Comments Graphite 260 Wh/kg Tesla The dominant negative electrode material used in lithium-ion batteries, limited to a capacity of 372 mAh/g. Low cost and good energy density. Graphite anodes can accommodate one lithium atom for every six carbon atoms. Charging rate is governed by the shape of the long, thin graphene sheets that constitute graphite. While charging, the lithium ions must travel to the outer edges of the graphene sheet before coming to rest (intercalating) between the sheets. The circuitous route takes so long that they encounter congestion around those edges. Lithium titanatePure Si can present a capacity density around mAh/g, but it will undergo a severe volume expansion (>300%), so it often being mixed with graphite. Another approach used carbon-coated 15 nm thick crystal silicon flakes. The tested half-cell achieved mAh/g over 800 cycles.
As graphite is limited to a maximum capacity of 372 mAh/g much research has been dedicated to the development of materials that exhibit higher theoretical capacities and overcoming the technical challenges that presently encumber their implementation. The extensive Review Article by Kasavajjula et al. summarizes early research on silicon-based anodes for lithium-ion secondary cells. In particular, Hong Li et al. showed in that the electrochemical insertion of lithium ions in silicon nanoparticles and silicon nanowires leads to the formation of an amorphous Li–Si alloy. The same year, Bo Gao and his doctoral advisor, Professor Otto Zhou described the cycling of electrochemical cells with anodes comprising silicon nanowires, with a reversible capacity ranging from at least approximately 900 to mAh/g.
Diamond-like carbon coatings can increase retention capacity by 40% and cycle life by 400% for lithium based batteries.
To improve the stability of the lithium anode, several approaches to installing a protective layer have been suggested. Silicon is beginning to be looked at as an anode material because it can accommodate significantly more lithium ions, storing up to 10 times the electric charge, however this alloying between lithium and silicon results in significant volume expansion (ca. 400%), which causes catastrophic failure for the cell. Silicon has been used as an anode material but the insertion and extraction of Li + {\\displaystyle {\\ce {\\scriptstyle Li+}}} can create cracks in the material. These cracks expose the Si surface to an electrolyte, causing decomposition and the formation of a solid electrolyte interphase (SEI) on the new Si surface (crumpled graphene encapsulated Si nanoparticles). This SEI will continue to grow thicker, deplete the available Li + {\\displaystyle {\\ce {\\scriptstyle Li+}}} , and degrade the capacity and cycling stability of the anode.
In addition to carbon- and silicon- based anode materials for lithium-ion batteries, high-entropy metal oxide materials are being developed. These conversion (rather than intercalation) materials comprise an alloy (or subnanometer mixed phases) of several metal oxides performing different functions. For example, Zn and Co can act as electroactive charge-storing species, Cu can provide an electronically conducting support phase and MgO can prevent pulverization.
Liquid electrolytes in lithium-ion batteries consist of lithium salts, such as LiPF
6, LiBF
4 or LiClO
4 in an organic solvent, such as ethylene carbonate, dimethyl carbonate, and diethyl carbonate. A liquid electrolyte acts as a conductive pathway for the movement of cations passing from the negative to the positive electrodes during discharge. Typical conductivities of liquid electrolyte at room temperature (20 °C (68 °F)) are in the range of 10 mS/cm, increasing by approximately 30–40% at 40 °C (104 °F) and decreasing slightly at 0 °C (32 °F). The combination of linear and cyclic carbonates (e.g., ethylene carbonate (EC) and dimethyl carbonate (DMC)) offers high conductivity and solid electrolyte interphase (SEI)-forming ability. While EC forms a stable SEI, it is not a liquid at room temperature, only becoming a liquid with the addition of additives such as the previously mentioned DMC or diethyl carbonate (DEC) or ethyl methyl carbonate (EMC). Organic solvents easily decompose on the negative electrodes during charge. When appropriate organic solvents are used as the electrolyte, the solvent decomposes on initial charging and forms a solid layer called the solid electrolyte interphase, which is electrically insulating, yet provides significant ionic conductivity, behaving as a solid electrolyte. The interphase prevents further decomposition of the electrolyte after the second charge as it grows thick enough to prevent electron tunneling after the first charge cycle. For example, ethylene carbonate is decomposed at a relatively high voltage, 0.7 V vs. lithium, and forms a dense and stable interface. Composite electrolytes based on POE (poly(oxyethylene)) provide a relatively stable interface. It can be either solid (high molecular weight) and be applied in dry Li-polymer cells, or liquid (low molecular weight) and be applied in regular Li-ion cells. Room-temperature ionic liquids (RTILs) are another approach to limiting the flammability and volatility of organic electrolytes.
The term solid electrolyte interphase was first coined by Peled in to describe the layer of insoluble products deposited on alkali and alkaline earth cathodes in non-aqueous batteries (NAB). However, Dey and Sullivan had noted previously in that graphite, in a lithium metal half cell using propylene carbonate (PC), reduced the electrolyte during discharge at a rate which linearly increased with the current. They proposed that the following reaction was taking place:
The same reaction was later proposed by Fong et al in , where they theorized that the carbonate ion was reacting with the lithium to form lithium carbonate, which was then forming a passivating layer on the surface of the graphite. PC is no longer used in batteries today as the molecules can intercalate into the graphite layers and react with the lithium there to form propylene and acts to delaminate the graphite.
The insulating properties of the SEI allow the battery to reach more extreme voltage gaps without simply reducing the electrolyte. This ability of the SEI to improve the voltage window of batteries was discovered almost on accident, but plays a vital role in high voltage batteries today.
Recent advances in battery technology involve using a solid as the electrolyte material. The most promising of these are ceramics. Solid ceramic electrolytes are mostly lithium metal oxides, which allow lithium-ion transport through the solid more readily due to the intrinsic lithium. The main benefit of solid electrolytes is that there is no risk of leaks, which is a serious safety issue for batteries with liquid electrolytes. Solid ceramic electrolytes can be further broken down into two main categories: ceramic and glassy. Ceramic solid electrolytes are highly ordered compounds with crystal structures that usually have ion transport channels. Common ceramic electrolytes are lithium super ion conductors (LISICON) and perovskites. Glassy solid electrolytes are amorphous atomic structures made up of similar elements to ceramic solid electrolytes but have higher conductivities overall due to higher conductivity at grain boundaries. Both glassy and ceramic electrolytes can be made more ionically conductive by substituting sulfur for oxygen. The larger radius of sulfur and its higher ability to be polarized allow higher conductivity of lithium. This contributes to conductivities of solid electrolytes are nearing parity with their liquid counterparts, with most on the order of 0.1 mS/cm and the best at 10 mS/cm. An efficient and economic way to tune targeted electrolytes properties is by adding a third component in small concentrations, known as an additive. By adding the additive in small amounts, the bulk properties of the electrolyte system will not be affected whilst the targeted property can be significantly improved. The numerous additives that have been tested can be divided into the following three distinct categories: (1) those used for SEI chemistry modifications; (2) those used for enhancing the ion conduction properties; (3) those used for improving the safety of the cell (e.g. prevent overcharging).[citation needed]
Electrolyte alternatives have also played a significant role, for example the lithium polymer battery. Polymer electrolytes are promising for minimizing the dendrite formation of lithium. Polymers are supposed to prevent short circuits and maintain conductivity.
The ions in the electrolyte diffuse because there are small changes in the electrolyte concentration. Linear diffusion is only considered here. The change in concentration c, as a function of time t and distance x, is
In this equation, D is the diffusion coefficient for the lithium ion. It has a value of 7.5×10−10 m2/s in the LiPF
6 electrolyte. The value for ε, the porosity of the electrolyte, is 0.724.
Lithium-ion batteries may have multiple levels of structure. Small batteries consist of a single battery cell. Larger batteries connect cells in parallel into a module and connect modules in series and parallel into a pack. Multiple packs may be connected in series to increase the voltage.
Batteries may be equipped with temperature sensors, heating/cooling systems, voltage regulator circuits, voltage taps, and charge-state monitors. These components address safety risks like overheating and short circuiting.
On the macrostructral level (length scale 0.1–5 mm) almost all commercial lithium-ion batteries comprise foil current collectors (aluminium for cathode and copper for anode). Copper is selected for the anode, because lithium does not alloy with it. Aluminum is used for the cathode, because it passivates in LiPF6 electrolytes.
Li-ion cells are available in various form factors, which can generally be divided into four types:
Cells with a cylindrical shape are made in a characteristic "swiss roll" manner (known as a "jelly roll" in the US), which means it is a single long "sandwich" of the positive electrode, separator, negative electrode, and separator rolled into a single spool. The result is encased in a container. One advantage of cylindrical cells is faster production speed. One disadvantage can be a large radial temperature gradient at high discharge rates.
The absence of a case gives pouch cells the highest gravimetric energy density; however, many applications require containment to prevent expansion when their state of charge (SOC) level is high, and for general structural stability. Both rigid plastic and pouch-style cells are sometimes referred to as prismatic cells due to their rectangular shapes. Three basic battery types are used in s-era electric vehicles: cylindrical cells (e.g., Tesla), prismatic pouch (e.g., from LG), and prismatic can cells (e.g., from LG, Samsung, Panasonic, and others).
Lithium-ion flow batteries have been demonstrated that suspend the cathode or anode material in an aqueous or organic solution.
As of , the smallest Li-ion cell was pin-shaped with a diameter of 3.5 mm and a weight of 0.6 g, made by Panasonic. A coin cell form factor is available for LiCoO2 cells, usually designated with a "LiR" prefix.
The average voltage of LCO (lithium cobalt oxide) chemistry is 3.6v if made with hard carbon cathode and 3.7v if made with graphite cathode. Comparatively, the latter has a flatter discharge voltage curve.: 25–26
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