UC San Diego

The sluggish progress of battery technologies has drastically hindered the rapid development of electric vehicles and next-generation portable electronics. Improving the energy density requires breakthroughs in materials for both cathode and anode, and new characterization methods to accurately correlate the materials with their performances.

For cathodes, lithium (Li) rich layered oxides exhibit high reversible specific capacities over 300 mAh g-1, attributing to the oxygen redox reaction. However, oxygen activity comes with instability in the form of oxygen loss, which is associated with irreversible voltage decay and capacity fading. Calculations suggest that incorporating 4d elements, such as Mo, enhances the structural stability by altering the local band structure and impeding oxygen vacancy formation. Driven by these findings, Mo is co-doped with Co into Li[Li0.2Ni0.2Mn0.6]O2, showing notably reduced voltage decay and capacity fading without sacrificing energy density and cycle life.

The Li metal anode is critical to break the energy-density bottleneck of current Li-ion chemistry. Inactive Li formation is the immediate cause of capacity loss and catastrophic failure of Li metal batteries. However, its composition has not yet been quantitatively studied due to the lack of effective diagnosis tools that can accurately differentiate Li+ in solid electrolyte interphase (SEI) components and the electrically isolated unreacted metallic Li0, which together comprise the inactive Li. By establishing a new analytical method, Titration Gas Chromatography (TGC), we accurately quantify the contribution from unreacted metallic Li0 to the total amount of inactive Li. We identify the Li0, rather than the (electro)chemically formed Li+ in SEI, as the dominating cause for the inactive Li and capacity loss. Coupling the measurements of the unreacted metallic Li0 global content to the observations of its local micro- and nano-structure by cryogenic electron microscopies, we also reveal the formation mechanism of inactive Li in different types of electrolytes, and determine the true underlying cause of low CE in Li metal deposition and stripping. We ultimately propose strategies for highly efficient Li deposition and stripping to enable Li metal anode for next generation high-energy batteries.