A lot of research has been done to improve lithium-ion battery safety, cycle life and power output over a range of high and low temperatures, yet understanding the fundamental processes and degradation mechanisms in Li-ion batteries remains a challenge.
To understand the degradation processes of lithium-ion batteries, it is important to understand how they operate. A typical cell is made up of an anode, cathode, electrolyte and separator. The anode is composed of carbon or graphite, the cathode of a lithium metal oxide and the electrolyte of a lithium-ion salt in a non-aqueous (aprotic) solvent. The separator is a permeable membrane that enables the lithium ions to shuttle between the anode and cathode, and it keeps the two electrodes from making contact and causing a short circuit.
During discharge, lithium ions migrate from the anode to the cathode. Electrons move in an external circuit in the same direction as the lithium ions, and current moves in the opposite direction. The discharging process is referred to as de-intercalation. During charging, an external electrical power source, or house current, applies an over-voltage that forces the lithium ions to pass in the reverse direction. This process is referred to intercalation. During intercalation, lithium ions move from an ordered lithium metal cathode lattice and become embedded between graphene sheets in the anode.
The solid electrolyte interface (SEI) enables the battery to operate in an efficient and reversible manner. The SEI film is composed of electrolyte reduction products that start forming on the surface of the anode during the initial battery charge. The SEI film functions as an ionic conductor that enables lithium to migrate through the film during intercalation and de-intercalation over the life of the battery. At the same time, the film also serves as an inductively passive electronic insulator which prevents further electrolyte reduction products forming on the anode, under typical operating conditions.
While capable of operating efficiently for years, Li-ion batteries can begin to fail prematurely when exposed to atypical conditions such as elevated temperature, charge effects or the presence of trace contaminants. These effects can initiate irreversible cell degradation, resulting in a loss of energy density, cycle life and safety.
The following five exothermic degradation reactions can occur between cell components:
- Chemical reduction of the electrolyte by the anode
- Thermal decomposition of the electrolyte
- Chemical oxidation of the electrolyte by the cathode
- Thermal decomposition by the cathode and anode
- Internal short circuit by charge effects
The SEI film that forms on the anode is composed of a mixture of inorganic and organic reduction products. These include lithium oxide, lithium fluoride and semicarbonates (e.g. lithium alkyl carbonates). Under typical conditions, such as room temperature and the absence of charge effects and contaminants, the SEI reaches a fixed film thickness, and the LIB can operate reversibly for years.
Chemical Reduction of the Electrolyte by the Anode
At elevated temperatures, alkyl carbonates on the SEI decompose into insoluble Li2CO3 that can increase the film thickness of the SEI layer, clogging the pores on the carbon surface and limiting accessibility of lithium ions to the anode surface. Inhibiting intercalation leads to an increase in impedance and eventually a loss in battery capacity, also referred to as capacity fade. Gases formed by the decomposition of the electrolyte increase the internal pressure in the cell and raise potential safety issues in sensitive environments.
Extended storage of the LIB is another condition that results in an incremental increase in SEI film thickness and capacity fade.
Over-charging & over-discharging
Over-charging the LIB (over 4.2 V) can initiate the reduction of Li+ on the anode as lithium plates, resulting in irreversible capacity fade. The randomness of the metallic lithium embedded in the anode during intercalation results in the formation of dendrites. Over time the dendrites can accumulate and pierce the separator, causing a short circuit between the electrodes and leading to a release of heat, and possibly fire and/or explosion. This process is referred to as thermal runaway.
Over-discharging (under 2 V) can also result in capacity fade. The anode copper current collector (a less commonly referenced battery component used to facilitate electron transfer) can dissolve into the electrolyte when the LIB is discharged. However, when charged, the copper ions can reduce on the anode as metallic copper in addition to the copper collector. Over time, metallic copper dendrites can form and lead to a short circuit in the same manner as metallic lithium dendrites.
Structural Disorder of the Anode
The anode is composed of materials that have a high surface area and provide large discharge and charge capacity. Cycling the battery at a high cycling rate and at a high state of charge induces mechanical strain on the graphite lattice from a high concentration of lithium ions packed between the graphene sheets. The mechanical strain caused by the insertion and de-insertion results in fissures and splits of the graphite particles, making them less oriented as compared to the original graphite particles. A relative change in the orientation of the graphite particles affects the reversible capacity of the anode and results in capacity fade.
The majority of electrolytes used in LIBs are composed of a lithium hexafluorophosphate (LiPF6) electrolyte in a solvent mixture of linear and cyclic carbonates (e.g. ethylene carbonate [EC] and dimethyl carbonate [DMC]). The combination of LiPF6 and carbonates is selected because of their high conductivity and SEI-forming ability. A mixture of carbonate solvents is needed to satisfy the requirement for high ionic conductivity (to dissolve and coordinate the lithium salt ions) and low viscosity (where the solvated ions occupy a small volume). As the two properties are mutually exclusive in a single carbonate solvent, the requirement is satisfied by mixing a high ionic-conductivity solvent with a low-viscosity solvent.
Electrolyte degradation mechanisms include hydrolysis and thermal decomposition.
Water is a major concern in LIBs. At concentrations as low as 10 ppm, water begins catalyzing a host of degradation products that can affect the electrolyte, anode and cathode.
Under typical conditions the electrolyte LiPF6 provides an ionic medium, enabling Li+ to shuttle between the electrodes during intercalation and de-intercalation. However, LiPF6 also participates in an equilibrium reaction with LiF and PF5.
Under typical conditions, the equilibrium lies far to the left. However, in the presence of water, the equilibrium reaction starts shifting to the right to form LiF, an insoluble, electronically insulating product. LiF forms on the surface of the anode resulting in an increase in SEI film thickness. As the SEI film thickens, it gives rise to an increase in impedance that can ultimately lead to capacity fade.
The hydrolysis of LiPF6 also yields PF5, a strong Lewis acid that reacts with electron-rich species such as water. Phosphorus pentafluoride reacts with water to form hydrofluoric acid and phosphorus oxyfluoride. Phosphorus oxyfluoride can in turn react with a second equivalent of water to form an additional quantity of hydrofluoric acid and the byproduct difluorohydroxy phosphoric acid.
The presence of hydrofluoric acid converts the rigid SEI film into a fragile one. In the case of the SEI layer that forms on the cathode, the carbonate solvent can diffuse onto the surface of cathode oxide over time, causing the release of heat and a possible thermal runaway condition.
Thermal Degradation of the Electrolyte
Carbonate-based LIBs, while effective at forming efficient SEI and providing power requirements, suffer from thermal decomposition. Decomposition of electrolyte salts and interactions between the salts and solvent start at as low as 70 degrees C. Significant decomposition occurs at higher temperatures. At 85 degrees C, transesterification products, such as dimethyl-2,5-dioxahexane carboxylate (DMDOHC) are formed from EC reacting with DMC.
In addition to liquid electrolytes, solid electrolytes are also in commercial use. Solid-polymer electrolytes (SPE) offer low environmental impact, are not as toxic as their liquid counterparts, are relatively low-cost, and remove any risk of flammable electrolyte and carbonate mixtures leaking out of the battery.
However, there are two inherent disadvantages: low ionic conductivity and low lithium transference. The low ionic conductivity results from poor salt dissociation. SPE is reported to dissociate as ion pairs. The low lithium transference results from stronger interaction of the polymer matrix with the lithium cation as compared to the anion.
Studies show that immobilizing the anion with additives produces a relative increase in lithium transference and ionic conductivity. Results from the analysis of an anion receptor, lithium triflate (LiCF3SO3) attached to an SPE composite by Raman spectroscopy show the distribution of electrolyte components in the polymer.
Two of the most commonly studied cathode materials are lithium cobalt oxide (LiCoO2) and lithium manganese oxide (LiMn2O4). LiCoO2 is the most widely used cathode material. LiMnO4 is considered a suitable alternative because of its low cost and ease of preparation, but its relatively poor cycling and storage capabilities have prevented it from being considered as a commercial replacement.
Cathode degradation mechanisms include manganese dissolution, chemical oxidation of the electrolyte by the cathode and structural disorder of the cathode.
The dissolution of manganese into the electrolyte is reported to occur as a result of hydrofluoric catalyzing the loss of metallic manganese through disproportionation of trivalent manganese, shown below.
Material loss of the spinel results in capacity fade. Thermal effects can also result in decrease in LIB performance. Temperatures as low as 50 degrees C initiate the deposition of the Mn2+ on the anode as metallic manganese (similar to lithium and copper plating), leading to an increase in impedance, a loss in battery capacity and potential thermal runaway. Cycling the LIB over the theoretical maximum and minimum voltage plateaus also results in severe capacity fade, due to destruction of the crystal lattice from Jahn-Teller distortion, which occurs when [Mn4+ is reduced to Mn3+] during discharge.
Electrolyte oxidation by the cathode
Storage of an overcharged LIB (over 3.6 V) initiates electrolyte oxidation by the cathode and induces formation of an SEI film on the cathode. As observed with the anode, excessive formation of the SEI on the cathode serves as an insulator, resulting in capacity fade, and can also lead to uneven current distribution.
Storage of an undercharged LIB (under 2 V) results in the slow degradation of LiCoO2 and LiMn2O4 cathodes, the release of oxygen and irreversible capacity loss.
With the growing demand for LIBs comes the expectation of improvements in battery performance and safety. These expected improvements include higher power output, minimal capacity loss, and extended battery life over extremes in temperature, charging and storage conditions.
One of the most significant improvements to the performance of LIBs, the SEI layer, is also one of its key weaknesses. The SEI layer is composed of electrolyte-carbonate reduction products that serve both as an ionic conductor and electronic insulator between the electrolyte and the electrode, but as results show, it is prone to thermal degradation. Formation of the thin layer on the anode and cathode has been the subject of great interest, as it determines many performance parameters of the battery. But, as the layer is formed after the battery has been assembled, it is difficult to analyze in-situ, making ex-situ analysis the only practical alternative. As a result, there are still many unanswered questions regarding SEI formation, composition and decomposition.
Advances in battery technology will be required to meet the growing demand for Li-ion batteries. To build higher-performance batteries, a variety of instruments and technologies will be needed to effectively understand degradation processes for each component individually and how they interact as a system.
Paul Voelker is Vertical Marketing Manager – Environmental & Industrial Markets for Thermo Fisher Scientifics’ chromatography and spectrometry products.
This article originally appeared in Charged Issue 14 – June/July 2014