An Inorganic-Rich Solid Electrolyte Interphase for Advanced Lithium-Metal Batteries in Carbonate Electrolytes.

In carbonate electrolytes, the organic-inorganic solid electrolyte interphase (SEI) formed on the Li-metal anode surface is strongly bonded to Li and experiences the same volume change as Li, thus it undergoes continuous cracking/reformation during plating/stripping cycles. Here, an inorganic-rich SEI is designed on a Li-metal surface to reduce its bonding energy with Li metal by dissolving 4m concentrated LiNO3 in dimethyl sulfoxide (DMSO) as an additive for a fluoroethylene-carbonate (FEC)-based electrolyte. Due to the aggregate structure of NO3 - ions and their participation in the primary Li+ solvation sheath, abundant Li2 O, Li3 N, and LiNx Oy grains are formed in the resulting SEI, in addition to the uniform LiF distribution from the reduction of PF6 - ions. The weak bonding of the SEI (high interface energy) to Li can effectively promote Li diffusion along the SEI/Li interface and prevent Li dendrite penetration into the SEI. As a result, our designed carbonate electrolyte enables a Li anode to achieve a high Li plating/stripping Coulombic efficiency of 99.55 % (1 mA cm-2 , 1.0 mAh cm-2 ) and the electrolyte also enables a Li||LiNi0.8 Co0.1 Mn0.1 O2 (NMC811) full cell (2.5 mAh cm-2 ) to retain 75 % of its initial capacity after 200 cycles with an outstanding CE of 99.83 %.

Enabling safe aqueous lithium ion open batteries by suppressing oxygen reduction reaction.

Due to the non-flammable nature of water-based electrolytes, aqueous lithium-ion batteries are resistant to catching fire. However, they are not immune to the risk of explosion, since the sealing structure adopted by current batteries limits the dissipation of heat and pressure within the cells. Here, we report a safe aqueous lithium-ion battery with an open configuration using water-in-salt electrolytes and aluminum oxide coated anodes. The design can inhibit the self-discharge by substantially suppressing the oxygen reduction reaction on lithiated anodes and enable good cycle performance over 1000 times. Our study may open a pathway towards safer lithium-ion battery designs.

A Highly Reversible, Dendrite-Free Lithium Metal Anode Enabled by a Lithium-Fluoride-Enriched Interphase.

Metallic lithium is the most competitive anode material for next-generation lithium (Li)-ion batteries. However, one of its major issues is Li dendrite growth and detachment, which not only causes safety issues, but also continuously consumes electrolyte and Li, leading to low coulombic efficiency (CE) and short cycle life for Li metal batteries. Herein, the Li dendrite growth of metallic lithium anode is suppressed by forming a lithium fluoride (LiF)-enriched solid electrolyte interphase (SEI) through the lithiation of surface-fluorinated mesocarbon microbeads (MCMB-F) anodes. The robust LiF-enriched SEI with high interfacial energy to Li metal effectively promotes planar growth of Li metal on the Li surface and meanwhile prevents its vertical penetration into the LiF-enriched SEI from forming Li dendrites. At a discharge capacity of 1.2 mAh cm-2 , a high CE of >99.2% for Li plating/stripping in FEC-based electrolyte is achieved within 25 cycles. Coupling the pre-lithiated MCMB-F (Li@MCMB-F) anode with a commercial LiFePO4 cathode at the positive/negative (P/N) capacity ratio of 1:1, the LiFePO4 //Li@MCMB-F cells can be charged/discharged at a high areal capacity of 2.4 mAh cm-2 for 110 times at a negligible capacity decay of 0.01% per cycle.

Azo Compounds Derived from Electrochemical Reduction of Nitro Compounds for High Performance Li-Ion Batteries.

Organic compounds are desirable alternatives for sustainable lithium-ion battery electrodes. However, the electrochemical properties of state-of-the-art organic electrodes are still worse than commercial inorganic counterparts. Here, a new chemistry is reported based on the electrochemical conversion of nitro compounds to azo compounds for high performance lithium-ion batteries. 4-Nitrobenzoic acid lithium salt (NBALS) is selected as a model nitro compound to systemically investigate the structure, lithiation/delithiation mechanism, and electrochemical performance of nitro compounds. NBALS delivers an initial capacity of 153 mAh g-1 at 0.5 C and retains a capacity of 131 mAh g-1 after 100 cycles. Detailed characterizations demonstrate that during initial electrochemical lithiation, the nitro group in crystalline NBALS is irreversibly reduced into an amorphous azo compound. Subsequently, the azo compound is reversibly lithiated/delithiated in the following charge/discharge cycles with high electrochemical performance. The lithiation/delithiation mechanism of azo compounds is also validated by directly using azo compounds as electrode materials, which exhibit similar electrochemical performance to nitro compounds, while having a much higher initial Coulombic efficiency. Therefore, this work proves that nitro compounds can be electrochemically converted to azo compounds for high performance lithium-ion batteries.

Existence of Solid Electrolyte Interphase in Mg Batteries: Mg/S Chemistry as an Example.

Magnesium redox chemistry is a very appealing "beyond Li ion chemistry" for realizing high energy density batteries due to the high capacity, low reduction potential, and most importantly, highly reversible and dendrite-free Mg metal anode. However, the progress of rechargeable Mg batteries has been greatly hindered by shortage of electrolytes with wide stability window, high ionic conductivity, and good compatibility with cathode materials. Unlike solid electrolyte interphase on Li metal anode, surface film formed by electrolyte decomposition in Mg batteries was considered to block Mg ion transport and passivate Mg electrode. For this reason, the attention of the community has been mainly focusing on surface layer free electrolytes, while reductively unstable salts/solvents are barely considered, despite many of them possessing all the necessary properties for good electrolytes. Here, for the first time, we demonstrate that the surface film formed by electrolyte decomposition can function as a solid electrolyte interphase (SEI). Using Mg/S chemistry as a model system, the SEI formation mechanism on Mg metal anode was thoroughly examined using electrochemical methods and surface chemistry characterization techniques such as EDX and XPS. On the basis of these results, a comprehensive view of the Mg/electrolyte interface that unifies both the SEI mechanism and the passivation layer mechanism is proposed. This new picture of surface layer on Mg metal anode in Mg batteries not only revolutionizes current understanding of Mg/electrolyte interface but also opens new avenues for electrolyte development by uncovering the potential of those reductively unstable candidates through interface design.

Self-Templated Formation of P2-type K0.6CoO2 Microspheres for High Reversible Potassium-Ion Batteries.

Layered metal oxides have been widely used as the best cathode materials for commercial lithium-ion batteries and are being intensively explored for sodium-ion batteries. However, their application to potassium-ion batteries (PIBs) is hampered because of the poor cycling stability and low rate capability due to the larger ionic size of K+ than of Li+ or Na+. Herein, a facile self-templated strategy was used to synthesize unique P2-type K0.6CoO2 microspheres that consist of aggregated primary nanoplates as PIB cathodes. The unique K0.6CoO2 microspheres with aggregated structure significantly enhanced the kinetics of the K+ intercalation/deintercation and also minimized the parasitic reactions between the electrolyte and K0.6CoO2. The P2-K0.6CoO2 microspheres demonstrated a high reversible capacity of 82 mAh g-1 at 10 mA g-1, high rate capability of 65 mAh g-1 at 100 mA g-1, and long cycle life (87% capacity retention over 300 cycles). The high reversibility of the P2-K0.6CoO2 full cell paired with a hard carbon anode further demonstrated the feasibility of PIBs. This work not only successfully demonstrates exceptional performance of P2-type K0.6CoO2 cathodes and microspheres K0.6CoO2∥hard carbon full cells, but also provides new insights into the exploration of other layered metal oxides for PIBs.