Using advanced imaging techniques, scientists at the Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab) have been able to observe what exactly happens inside a cathode particle as lithium-ion batteries are charged and discharged. The team, led led by Berkeley Lab materials chemist Guoying Chen, uncovered important insights into reactions in cathode materials, including the discovery of particle cracking as the cathode is charged, which can reduce battery capacity and life.
The study, published as an open-access paper in the journal Nature Communications, used advanced two-dimensional and three-dimensional nano-tomography on a series of well-formed LixMn1.5Ni0.5O4 (0≤x≤1) crystals to visualize the mesoscale phase distribution, as a function of Li content at the sub-particle level.
Solid-state phase transformation is one of the key processes in the Li-ion battery technology. Conventional wisdom suggests that first-order phase transition involving large lattice misfit between the phases often leads to low-rate capability in battery cycling, yet some electrode materials, notably, nano-LiFePO4 (LFP) and spinel LiMn1.5Ni0.5O4 (LMNO), can be cycled at very high rates even in the presence of two-phase reactions with large volume changes (ca. 7 and 6.3% for LFP and LMNO, respectively). Much effort has been dedicated to the understanding of phase transition mechanism in LFP that led to the discovery of the non-equilibrium solid–solution pathway that bypasses the nucleation and growth process in nano-sized LFP. Despite its technological significance as a promising cathode materials for high-energy Li-ion batteries, very few studies focused on detailed phase transformation in LMNO. High-rate capability can be achieved even on micron-sized LMNO, but the relationships among kinetics, strain and phase boundary movement are currently unknown.
… In this work, we prepare a series of micron-sized (with an average size of 3 μm) and octahedron-shaped LixMn1.5Ni0.5O4 (LxMNO) crystals with dominant (111) family surface facets and employe FF-TXM-XANES to visualize the mesoscale phase distribution at a single-crystallite level for the first time.—Kuppan et al.
Understanding dynamic reaction pathways in solid matter and the phase transformation mechanism is extremely difficult but critical in designing advanced materials—not just battery cathodes but materials for other applications as well. The uniqueness of this work was the combination of using high-resolution two-dimensional and three-dimensional imaging techniques on single-crystal materials.—Guoying Chen
Chen chose to use a lithium manganese nickel oxide cathode because it is viewed as one of the next-generation battery materials. The material’s higher energy comes from its unique high charge and discharge voltage; however, the high voltage also provokes enhanced reactivity from the electrolyte and leads to a less stable battery, Chen said.
The researchers mapped out the chemical and phase distribution on their particles at a very high spatial resolution. These maps provide “fossil evidence” of the phase transformation, which allowed them to achieve unprecedented mechanistic understanding of the electrode material.
What they saw was a unique nucleation and growth process involving multiple phases simultaneously on the same particle. The impact of the volume differences between the phases, a reduction of more than 6% in total, caused the particles to crack. This becomes more significant as the particle approaches the fully delithiated state.
The cracking, Chen asserts, is likely one of the leading causes of the fade in long-term battery cycling that researchers have seen with this cathode.
If you have cracking, it means fresh surface keeps getting exposed, thus causing more reactions with the electrolyte, which consumes the electrolyte and reduces the lifetime of the battery. If we can minimize or eliminate the cracking issue, we probably will see much improved stability.—Guoying Chen
The researchers are examining two ideas to minimize the cracking, using smaller particles and avoiding fully charging the particles. Reducing particle size can be problematic as it also increases the surface area, which means more side reactions, Chen said. “It is important to find the optimal size.”
Chen’s group is also looking for other approaches for high-energy batteries, such as materials that can provide a high capacity.
The imaging was done at SLAC National Accelerator Laboratory’s Stanford Synchrotron Radiation Lightsource (SSRL), a Department of Energy (DOE) Office of Science User Facility. SSRL scientists Yahong Xu and Yijin Liu were co-authors of the paper, as was former Berkeley Lab postdoctoral fellow Saravanan Kuppan. The work was funded by DOE’s Energy Efficiency and Renewable Energy Office.
Saravanan Kuppan, Yahong Xu, Yijin Liu, Guoying Chen (2017) “Phase transformation mechanism in lithium manganese nickel oxide revealed by single-crystal hard X-ray microscopy.” Nature Communications 8: 14309 doi: 10.1038/NCOMMS14309