Materials for lithium-ion battery safety

Abstract

Lithium-ion batteries (LIBs) are considered to be one of the most important energy storage technologies. As the energy density of batteries increases, battery safety becomes even more critical if the energy is released unintentionally. Accidents related to fires and explosions of LIBs occur frequently worldwide. Some have caused serious threats to human life and health and have led to numerous product recalls by manufacturers. These incidents are reminders that safety is a prerequisite for batteries, and serious issues need to be resolved before the future application of high-energy battery systems. This Review aims to summarize the fundamentals of the origins of LIB safety issues and highlight recent key progress in materials design to improve LIB safety. We anticipate that this Review will inspire further improvement in battery safety, especially for emerging LIBs with high-energy density.

Fig. 1. Three stages for the thermal runaway process.

Stage 1: The onset of overheating. The batteries change from a normal to an abnormal state, and the internal temperature starts to increase. Stage 2: Heat accumulation and gas release process. The internal temperature quickly rises, and the battery undergoes exothermal reactions. Stage 3: Combustion and explosion. The flammable electrolyte combusts, leading to fires and even explosions.

Fig. 2. Typical self-heating rate tests by ARC during thermal ramp test of LIB.

The anode is mesocarbon microbead graphite. The cathode is LiNi_0.8Co_0.05Al_0.05O₂. The electrolyte is 1.2 M LiPF₆ in EC/PC/DMC. A Celgard 2325 trilayer separator was used. Adapted with permission from Electrochemical Society Inc.

Fig. 3. Strategies to solve the issues in stage 1.

(A) Shear thickening electrolyte. Top: For normal electrolyte, mechanical impact can lead to battery internal shorting, causing fires and explosions. Bottom: The novel smart electrolyte with shear thickening effect under pressure or impact demonstrates excellent tolerance to crushing, which could significantly improve the mechanical safety of batteries. (B) Bifunctional separators for early detection of lithium dendrites. Dendrite formation in a traditional lithium battery, where complete penetration of the separator by a lithium dendrite is only detected when the battery fails because of an internal short circuit. In comparison, a lithium battery with a bifunctional separator (consisting of a conducting layer sandwiched between two conventional separators), where the overgrown lithium dendrite penetrates the separator and makes contact with the conducting copper layer, resulting in a drop in V_Cu−Li, which serves as a warning of impending failure due to an internal short circuit. However, the full battery remains safely operational with nonzero potential. (A) and (B) are adapted or reproduced with permission from Springer Nature. (C) Trilayer separator to consume hazardous Li dendrites and extend battery life. Left: Lithium anodes can easily form dendritic deposits, which can gradually grow bigger and penetrate the inert polymer separator. When the dendrites finally connect the cathode and anode, the battery is short-circuited and fails. Right: A layer of silica nanoparticles was sandwiched by two layers of commercial polymer separators. Therefore, when lithium dendrites grow and penetrate the separator, they will contact the silica nanoparticles in the sandwiched layer and be electrochemically consumed. (D) Scanning electron microscopy (SEM) image of the silica nanoparticle sandwiched separator. (E) Typical voltage versus time profile of a Li/Li battery with a conventional separator (red curve) and the silica nanoparticle sandwiched trilayer separator (black curve) tested under the same conditions. (C), (D), and (E) are reproduced with permission from John Wiley and Sons. (F) Schematic illustration of the mechanisms of the redox shuttle additives. On an overcharged cathode surface, the redox additive is oxidized to the form [O], which subsequently would be reduced back to its original state [R] on the surface of the anode by diffusion through the electrolyte. The electrochemical cycle of oxidation-diffusion-reduction-diffusion can be maintained indefinitely and hence locks the cathode potential from hazardous overcharging. (G) Typical chemical structures of the redox shuttle additives. (H) Mechanism of the shutdown overcharge additives that can electrochemically polymerize at high potentials. (I) Typical chemical structures of the shutdown overcharge additives. The working potentials of the additives are listed under each molecular structure in (G), (H), and (I).

Fig. 4. Strategies to solve the issues in stage 2: Reliable cathodes.

(A) Schematic diagram of a positive electrode particle with a Ni-rich core surrounded by a concentration-gradient outer layer. Each particle has a Ni-rich central bulk Li(Ni_0.8Co_0.1Mn_0.1)O₂ and a Mn-rich outer layer [Li(Ni_0.8Co_0.1Mn_0.1)O₂] with decreasing Ni concentration and increasing Mn and Co concentrations as the surface is approached. The former provides high capacity, whereas the latter improves the thermal stability. The average composition is Li(Ni_0.68Co_0.18Mn_0.18)O₂. A scanning electron micrograph of a typical particle is also shown on the right. (B) Electron-probe x-ray microanalysis results of the final lithiated oxide Li(Ni_0.64Co_0.18Mn_0.18)O₂. The gradual concentration changes of Ni, Mn, and Co in the interlayer are evident. The Ni concentration decreases, and the Co and Mn concentrations increase toward the surface. (C) Differential scanning calorimetry (DSC) traces showing heat flow from the reaction of the electrolyte with concentration-gradient material Li(Ni_0.64Co_0.18Mn_0.18)O₂, the Ni-rich central material Li(Ni_0.8Co_0.1Mn_0.1)O₂, and the Mn-rich outer layer [Li(Ni_0.46Co_0.23Mn_0.31)O₂]. The materials were charged to 4.3 V. (A), (B), and (C) are reproduced with permission from Springer Nature. (D) Left: Transmission electron microscopy (TEM) bright-field image of the AlPO₄ nanoparticle–coated LiCoO₂; energy dispersive x-ray spectrometry confirms the Al and P components in the coating layer. Right: High-resolution TEM image showing the AlPO₄ nanoparticles (~3 nm in diameter) in the nanoscale coating layer; the arrows indicate the interface between the AlPO₄ layer and LiCoO₂. (E) Left: A picture of a cell containing a bare LiCoO₂ cathode after the 12-V overcharge test. The cell burned and exploded at that voltage. Right: A picture of a cell containing the AlPO₄ nanoparticle–coated LiCoO₂ after the 12-V overcharge test. (D) and (E) are reproduced with permission from John Wiley and Sons.

Fig. 5. Strategies to solve the issues in stage 2.

(A) Schematic illustration of the thermal switching mechanism of the TRPS current collector. The safe battery has one or two current collectors coated with a thin TRPS layer. It operates normally at room temperature. However, in case of high temperature or large current, the polymer matrix expands, thus separating the conductive particles, which can decrease its conductivity, greatly increasing its resistance and shutting down the battery. The battery structure can thus be protected without damage. On cooling, the polymer shrinks and regains the original conductive pathways. (B) Resistivity changes of different TRPS films as a function of temperature, including PE/GrNi with different GrNi loadings and PP/GrNi with a 30% (v/v) loading of GrNi. (C) Capacity summary of the safe LiCoO₂ battery cycling between 25°C and shutdown. The near-zero capacity at 70°C indicates full shutdown. (A), (B), and (C) are reproduced with permission from Springer Nature. (D) Schematic representation of microsphere-based shutdown concept for LIBs. Electrodes are functionalized with thermoresponsive microspheres that, above a critical internal battery temperature, undergo a thermal transition (melt). The molten capsules coat the electrode surface, forming an ionically insulating barrier and shutting down the battery cell. (E) A thin and self-standing inorganic composite membrane composed of 94% alumina particles and 6% styrene-butadiene rubber (SBR) binder was prepared by a solution casting method. Right: Photographs showing the thermal stability of the inorganic composite separator and the PE separator. The separators were held at 130°C for 40 min. The PE significantly shrank from the area with the dotted square. However, the composite separator did not show obvious shrinkage. Reproduced with permission from Elsevier. (F) Molecular structure of some high-melting temperature polymers as separator materials with low high-temperature shrinkage. Top: polyimide (PI). Middle: cellulose. Bottom: poly(butylene) terephthalate. (G) Left: Comparison of the DSC spectra of the PI with the PE and PP separator; the PI separator shows excellent thermal stability at the temperature range from 30° to 275°C. Right: Digital camera photos comparing the wettability of a commercial separator and the as-synthesized PI separator with a propylene carbonate electrolyte. Reproduced with permission from the American Chemical Society.

Fig. 6. Strategies to solve the issues in stage 3.

(A) Typical molecular structures of flame-retardant additives. (B) The mechanism for the flame retardation effects of these phosphorus-containing compounds is generally believed to be a chemical radical-scavenging process, which can terminate the radical chain reactions responsible for the combustion reaction in the gas phase. TPP, triphenyl phosphate. (C) The self-extinguish time (SET) of the typical carbonate electrolyte can be significantly reduced with the addition of triphenyl phosphate. (D) Schematic of the “smart” electrospun separator with thermal-triggered flame-retardant properties for LIBs. The free-standing separator is composed of microfibers with a core-shell structure, where the flame retardant is the core and the polymer is the shell. Upon thermal triggering, the polymer shell melts and then the encapsulated flame retardant is released into the electrolyte, thus effectively suppressing the ignition and burning of the electrolytes. (E) SEM image of the TPP@PVDF-HFP microfibers after etching clearly shows their core-shell structure. Scale bar, 5 μm. (F) Typical molecular structures of room temperature ionic liquid used as nonflammable electrolytes for LIBs. (G) The molecular structure of PFPE, a nonflammable perfluorinated PEO analog. Two methyl carbonate groups are modified on the terminals of polymer chains to ensure the compatibility of the molecules with current battery systems.