A high energy X-ray diffraction technique is employed in a new way to make operando through-thickness temperature and other measurements inside a large format commercial Li battery pouch cell. The technique, which has a sub-mm in-plane spatial resolution, simultaneously determines the local internal pouch temperature, the local state of charge of both electrodes (as opposed to the global average state of charge determined electrochemically), and the local in-plane elastic strain in the current collector, all without embedding any intrusive sensors that may alter battery behavior. The technique also allows determination of local thermal conductivity and local electrical resistance in lithium battery pouch cells. As both thermal strain and mechanical strain develop during the charge-discharge cycling of the pouch cell, a novel approach developed herein makes it possible to separate them, allowing for measurement of the local temperature inside the battery. The operando experiment reveals that the internal pouch temperature is substantially higher than the external pouch temperature. We propose that mechanical strain is due primarily to load transfer from the electrode to the current collector during lithiation. When there are significant temperature excursions, there is additional mechanical strain from the thermal expansion mismatch between electrodes and their current collectors. Detailed local SOC mapping illustrates non-uniform degradation of the battery pouch cell. The possibility for 3D measurements in cylindrical cells is proposed. We believe that this new approach can provide critically needed data for validation of detailed models of processes inside commercial lithium battery pouch cells.
XRD allows us to measure internal pouch T, which is much higher than external T. Here we plot how internal and external T increases at 20C as the pouch is charged.
The goal of much of present day Lithium battery research is to develop higher energy density batteries. Consider 4 approaches:
(1) Positive electrodes with greater capacity. Such electrodes can be made, but their durability is poor.
(2) Higher voltage positive electrodes, up to 5 V. Such electrodes can be made, but we do not have electrolytes that are stable at such high voltages. Neither approach addresses volumetric energy density.
(3) Higher mass density (lower porosity) electrodes. This could lead to higher tortuosity. A reduction in porosity from 40% to 25% would again increase energy density by 25%.
(4) More durable electrodes. The connection between durability and energy density comes from the fact that in order to achieve long life, much of the energy in Li-ion batteries is never accessed. If we could access 80% of the battery’s energy instead of, say, 65%, the energy density would increase by 25%. Importantly, both volumetric and gravimetric energy density would increase.
We believe that heterogeneity is the ultimate reason that many Li-ion batteries access only about 65% of their theoretical energy. That’s because when the average state of charge (SOC) in an electrode is 65%, parts of the electrode are already at 100% state of charge, and at such high values for SOC, either plating or electrolyte oxidation occurs. See Figure 6 of”Particle Size Polydispersity in Li-ion Batteries.”
Our work researching lithium battery problems is predicated on two hypotheses: First, that degradation and failure initiate at inhomogeneities (or heterogeneities) the the battery microstructure; and second, that these heterogeneities lead to an inhomogeneous transport of lithium ions. Inhomogeneities include any structures where there are rapidly varying spatial properties, such as the SEI lyer. The SEI film is an important site for generating lithium battery aging and failure. For this reason, we believe that a general study of degradation and failure can begin with identification and quantification of inhomogeneities (typically at the mesoscale) as well as measurements of Li transport and insertion into porous electrodes in the battery. These measurements could then guide researchers towards other experiments and models that provide fundamental knowledge of durability (aging, degradation and failure) using advanced diagnostic techniques. We are especially interested in research that connects with models that take into account microstructural and nanoscale properties and inhomogeneities.