Enhanced Interfacial Kinetics and High-Voltage/High-Rate Performance of LiCoO₂ Cathode by Controlled Sputter-Coating with a Nanoscale Li₄Ti₅O₁₂ Ionic Conductor

The selection and optimization of coating material/approach for electrode materials have been under intensive pursuit to address the high-voltage induced degradation of lithium ion batteries. Herein, we demonstrate an efficient way to enhance the high-voltage electrochemical performance of LiCoO₂ cathode by postcoating of its composite electrode with Li₄Ti₅O₁₂ (LTO) via magnetron sputtering. With a nanoscale (∼25 nm) LTO coating, the reversible capacity of LiCoO₂ after 60 cycles is significantly increased by 40% (to 170 mAh g^–1) at room temperature and by 118% (to 139 mAh g^–1) at 55 °C. Meanwhile, the electrode’s rate capability is also greatly improved, which should be associated with the high Li⁺ diffusivity of the LTO surface layer, while the bulk electronic conductivity of the electrode is unaffected. At 12 C, the capacity of the coated electrode reaches 113 mAh g^–1, being 70% larger than that of the uncoated one. The surface interaction between LTO and LiCoO₂ is supposed to reduce the space-charge layer at the LiCoO₂–electrolyte interface, which makes the Li⁺ diffusion much easier as evidenced by the largely enhanced diffusion coefficient of the coated electrode (an order of magnitude improvement). In addition, the LTO coating layer, which is electrochemically and structurally stable in the applied potential range, plays the role of a passivation layer or an artificial and friendly solid electrolyte interface (SEI) layer on the electrode surface. Such protection is able to impede propagation of the in situ formed irreversible SEI and thus guarantee a high initial columbic efficiency and superior cycling stability at high voltage

Multistep Lithiation of Tin Sulfide: An Investigation Using <i>in Situ</i> Electron Microscopy

Two-dimensional (2D) metal sulfides have been widely explored as promising electrodes for lithium-ion batteries since their two-dimensional layered structure allows lithium ions to intercalate between layers. For tin disulfide, the lithiation process proceeds <i>via</i> a sequence of three different types of reactions: intercalation, conversion, and alloying, but the full scenario of reaction dynamics remains nebulous. Here, we investigate the dynamical process of the multistep reactions using <i>in situ</i> electron microscopy and discover the formation of an intermediate rock-salt phase with the disordering of Li and Sn cations after initial 2D intercalation. The disordered cations occupy all the octahedral sites and block the channels for intercalation, which alter the reaction pathways during further lithiation. Our first-principles calculations of the nonequilibrium lithiation of SnS₂ corroborate the energetic preference of the disordered rock-salt structure over known layered polymorphs. The <i>in situ</i> observations and calculations suggest a two-phase reaction nature for intercalation, disordering, and following conversion reactions. In addition, <i>in situ</i> delithiation observation confirms that the alloying reaction is reversible, while the conversion reaction is not, which is consistent with the <i>ex situ</i> analysis. This work reveals the full lithiation characteristic of SnS₂ and sheds light on the understanding of complex multistep reactions in 2D materials

Multistep Lithiation of Tin Sulfide: An Investigation Using <i>in Situ</i> Electron Microscopy

Two-dimensional (2D) metal sulfides have been widely explored as promising electrodes for lithium-ion batteries since their two-dimensional layered structure allows lithium ions to intercalate between layers. For tin disulfide, the lithiation process proceeds <i>via</i> a sequence of three different types of reactions: intercalation, conversion, and alloying, but the full scenario of reaction dynamics remains nebulous. Here, we investigate the dynamical process of the multistep reactions using <i>in situ</i> electron microscopy and discover the formation of an intermediate rock-salt phase with the disordering of Li and Sn cations after initial 2D intercalation. The disordered cations occupy all the octahedral sites and block the channels for intercalation, which alter the reaction pathways during further lithiation. Our first-principles calculations of the nonequilibrium lithiation of SnS₂ corroborate the energetic preference of the disordered rock-salt structure over known layered polymorphs. The <i>in situ</i> observations and calculations suggest a two-phase reaction nature for intercalation, disordering, and following conversion reactions. In addition, <i>in situ</i> delithiation observation confirms that the alloying reaction is reversible, while the conversion reaction is not, which is consistent with the <i>ex situ</i> analysis. This work reveals the full lithiation characteristic of SnS₂ and sheds light on the understanding of complex multistep reactions in 2D materials

Multistep Lithiation of Tin Sulfide: An Investigation Using <i>in Situ</i> Electron Microscopy

Two-dimensional (2D) metal sulfides have been widely explored as promising electrodes for lithium-ion batteries since their two-dimensional layered structure allows lithium ions to intercalate between layers. For tin disulfide, the lithiation process proceeds <i>via</i> a sequence of three different types of reactions: intercalation, conversion, and alloying, but the full scenario of reaction dynamics remains nebulous. Here, we investigate the dynamical process of the multistep reactions using <i>in situ</i> electron microscopy and discover the formation of an intermediate rock-salt phase with the disordering of Li and Sn cations after initial 2D intercalation. The disordered cations occupy all the octahedral sites and block the channels for intercalation, which alter the reaction pathways during further lithiation. Our first-principles calculations of the nonequilibrium lithiation of SnS₂ corroborate the energetic preference of the disordered rock-salt structure over known layered polymorphs. The <i>in situ</i> observations and calculations suggest a two-phase reaction nature for intercalation, disordering, and following conversion reactions. In addition, <i>in situ</i> delithiation observation confirms that the alloying reaction is reversible, while the conversion reaction is not, which is consistent with the <i>ex situ</i> analysis. This work reveals the full lithiation characteristic of SnS₂ and sheds light on the understanding of complex multistep reactions in 2D materials

Multistep Lithiation of Tin Sulfide: An Investigation Using <i>in Situ</i> Electron Microscopy

Two-dimensional (2D) metal sulfides have been widely explored as promising electrodes for lithium-ion batteries since their two-dimensional layered structure allows lithium ions to intercalate between layers. For tin disulfide, the lithiation process proceeds <i>via</i> a sequence of three different types of reactions: intercalation, conversion, and alloying, but the full scenario of reaction dynamics remains nebulous. Here, we investigate the dynamical process of the multistep reactions using <i>in situ</i> electron microscopy and discover the formation of an intermediate rock-salt phase with the disordering of Li and Sn cations after initial 2D intercalation. The disordered cations occupy all the octahedral sites and block the channels for intercalation, which alter the reaction pathways during further lithiation. Our first-principles calculations of the nonequilibrium lithiation of SnS₂ corroborate the energetic preference of the disordered rock-salt structure over known layered polymorphs. The <i>in situ</i> observations and calculations suggest a two-phase reaction nature for intercalation, disordering, and following conversion reactions. In addition, <i>in situ</i> delithiation observation confirms that the alloying reaction is reversible, while the conversion reaction is not, which is consistent with the <i>ex situ</i> analysis. This work reveals the full lithiation characteristic of SnS₂ and sheds light on the understanding of complex multistep reactions in 2D materials