Large Local Currents in a Lithium-Ion Battery during Rest after Fast Charging.

Increasing electric vehicle (EV) adoption requires lithium-ion batteries that can be charged quickly and safely. Some EV batteries have caught on fire despite being neither charged nor discharged. While the lithium that plates on graphite during fast charging affects battery safety, so do the internal ionic currents that can occur when the battery is at rest after charging. These currents are difficult to quantify; the external current that can readily be measured is zero. Here we study a graphite electrode at rest after 6C fast charging using operando X-ray microtomography. We quantify spatially resolved current density distributions that originate at plated lithium and end in underlithiated graphite particles. The average current densities decrease from 1.5 to 0.5 mA cm-2 in about 20 min after charging is stopped. Surprisingly, the range of the stripping current density is independent of time, with outliers above 20 mA cm-2. The persistence of outliers provides a clue as to the origin of catastrophic failure in batteries at rest.

Exploring the Promise of Multifunctional "Zintl-Phase-Forming" Electrolytes for Si-Based Full Cells.

Li-M-Si ternary Zintl phases have gained attention recently due to their high structural stability, which can improve the cycling stability compared to a bulk Si electrode. Adding multivalent cation salts (such as Mg2+ and Ca2+) in the electrolyte was proven to be a simple way to form Li-M-Si ternary phases in situ in Si-based Li-ion cells. To explore the promise of Zintl-phase-forming electrolytes, we systematically investigated their application in pouch cells via electrochemical and multiscale postmortem analysis. The introduction of multivalent cations, such as Mg2+, during charging can form LixMySi ternary phases. They can stabilize Si anions and reduce side reactions with electrolyte, improving the bulk stability. More importantly, Mg2+ and Ca2+ incorporate into interfacial side reactions and generate inorganic-rich solid-electrolyte interphase, thus enhancing the interfacial stability. Therefore, the full cells with Zintl-phase-forming electrolytes achieve higher capacity retentions at the C/3 rate after 100 cycles, compared to a baseline electrolyte. Additionally, strategies for mitigating the electrode-level fractures of Si were evaluated to make the best use of Zintl-phase-forming electrolytes. This work highlights the significance of synergistic impact of multifunctional additives to stabilize both bulk and interface chemistry in high-energy Si anode materials for Li-ion batteries.

3D Detection of Lithiation and Lithium Plating in Graphite Anodes during Fast Charging.

A barrier to the widespread adoption of electric vehicles is enabling fast charging lithium-ion batteries. At normal charging rates, lithium ions intercalate into the graphite electrode. At high charging rates, lithiation is inhomogeneous, and metallic lithium can plate on the graphite particles, reducing capacity and causing safety concerns. We have built a cell for conducting high-resolution in situ X-ray microtomography experiments to quantify three-dimensional lithiation inhomogeneity and lithium plating. Our studies reveal an unexpected correlation between these two phenomena. During fast charging, a layer of mossy lithium metal plates at the graphite electrode-separator interface. The transport bottlenecks resulting from this layer lead to underlithiated graphite particles well-removed from the separator, near the current collector. These underlithiated particles lie directly underneath the mossy lithium, suggesting that lithium plating inhibits further lithiation of the underlying electrode.

Mn(II) deposition on anodes and its effects on capacity fade in spinel lithium manganate-carbon systems.

Dissolution and migration of manganese from cathode lead to severe capacity fading of lithium manganate-carbon cells. Overcoming this major problem requires a better understanding of the mechanisms of manganese dissolution, migration and deposition. Here we apply a variety of advanced analytical methods to study lithium manganate cathodes that are cycled with different anodes. We show that the oxidation state of manganese deposited on the anodes is +2, which differs from the results reported earlier. Our results also indicate that a metathesis reaction between Mn(II) and some species on the solid-electrolyte interphase takes place during the deposition of Mn(II) on the anodes, rather than a reduction reaction that leads to the formation of metallic Mn, as speculated in earlier studies. The concentration of Mn deposited on the anode gradually increases with cycles; this trend is well correlated with the anodes rising impedance and capacity fading of the cell.

New class of nonaqueous electrolytes for long-life and safe lithium-ion batteries.

Long-life and safe lithium-ion batteries have been long pursued to enable electrification of the transportation system and for grid applications. However, the poor safety characteristics of lithium-ion batteries have been the major bottleneck for the widespread deployment of this promising technology. Here, we report a novel nonaqueous Li(2)B(12)F(12-x)H(x) electrolyte, using lithium difluoro(oxalato)borate as an electrolyte additive, that has superior performance to the conventional LiPF(6)-based electrolyte with regard to cycle life and safety, including tolerance to both overcharge and thermal abuse. Cells tested with the Li(2)B(12)F(9)H(3)-based electrolyte maintained about 70% initial capacity when cycled at 55 °C for 1,200 cycles, and the intrinsic overcharge protection mechanism was active up to 450 overcharge abuse cycles. Results from in situ high-energy X-ray diffraction showed that the thermal decomposition of the delithiated Li(1-x)[Ni(1/3)Mn(1/3)Co(1/3)](0.9)O(2) cathode was delayed by about 20 °C when using the Li(2)B(12)F(12)-based electrolyte.