Li-Ion Battery Analysis Guide

Thermal Analysis of Lithium Ion Batteries

Innovative analytical solutions for thermal analysis can be used to test individual battery components, like anode/cathode electrode materials, separators, electrolytes, and more. Critical tools for the investigation of batteries' thermal stabilities, exothermic reactions, and enthalpies include differential scanning calorimetry (DSC), thermogravimetry (TGA), thermomechanical analysis (TMA), and dynamic mechanical analysis (DMA).

Risks associated with thermal runaway situations, such as overheating and possible explosion, are especially important for the use of lithium-ion batteries (LIBs) in EV applications. Battery safety is a key component for the further use of battery technology in our everyday life.

This application guide provides an overview of lithium-ion battery technology and demonstrates how various thermal analysis techniques can be employed for a host of R&D and QC applications.

The following application examples are provided:

Thermal stability of LiFePO₄ cathode material in electrolyte
Characterization of an electrolyte mixture
Analysis of microporous separators by TGA and TMA
Quality control of PVDF by TGA and DSC
Conversion of graphene oxide into graphene (anode material)

Common Applications for Thermal Analysis Techniques for Battery Components

To acquire more information about degradation components from a single experiment, a METTLER TOLEDO TGA or TGA/DSC can be hyphenated to a suitable gas analysis system. The new system can now perform evolved gas analysis (EGA). A TGA can be connected to a Fourier transform infrared spectroscopy, mass spectroscopy, gas chromatography-mass spectroscopy, or micro gas chromatography-mass spectroscopy (respectively FTIR spectroscopy, MS, GC/MS; Micro GC(/MS).

Basic Working Principle of a Li-ion Battery

LIBs consist of a positive electrode (cathode), negative electrode (anode), and electrolytic solution. When the cell is charging, the cathode (usually lithium cobalt oxide) is oxidized, and the anode (usually graphite) is reduced. When the cell is discharging, the reverse occurs. The Li+ ions do not partake in the overall electrochemical reaction and remain in their oxidized state. They travel between the anode and cathode by diffusion through a liquid electrolyte consisting of organic solvents, lithium salts, and various additives. The separator ensures the anode and cathode are kept electrically isolated but is porous enough to allow the electrolyte and Li+ ions to pass easily through it.

Electrodes (Anodes and Cathodes)
The performance and safety of electrodes are largely influenced by charge/discharge-induced aging and degradation of cathode active material. Providing precise measurements for heat capacity, decomposition temperatures, and enthalpy determination, thermal analysis techniques are fundamental aids in thermal stability studies.

Battery Separator
Separators for Li-ion batteries have a crucial impact on battery performance and life, as well as reliability and safety. They must be thin to allow Li+ ions to move quickly between the anode and cathode, but the structural integrity of the separator is important because its degradation could lead to an internal short circuit.

Thermal analysis is used to characterize the thermal properties of separators, typically made from polyolefins (e.g. PP or PE). Technological limitations of such membranes include penetration resistance, shrinkage, and meltdown. These properties can be investigated by means of Thermogravimetry (TGA), differential scanning calorimetry (DSC), and thermomechanical analysis (TMA).

Electrolytes
Differential scanning calorimetry (DSC) can be used in QC to study the composition and content of carbonates in electrolytic solutions, which have important implications for the cycling stability, energy density, and safety of lithium-ion batteries. DSC also provides information about electrolyte melting and crystallization for determining the minimum temperatures for charging/discharging processes.