Liubin Ben

Silicon-based nanosheets synthesized by a topochemical reaction for use as anodes for lithium ion batteries

Silicon is the most promising anode material for the next generation high-performance lithium ion batteries. However, its commercial application is hindered by its poor performance due to the huge volume change during cycling. Although two-dimensional silicon-based materials show significantly improved performance, flexible synthesis of such materials is still a challenge. In this work, silicon-based nanosheets with a multilayer structure are synthesized for the first time by a topochemical reaction. The morphology and oxidation state of these nanosheets can be controlled by appropriate choice of reaction media and oxidants. Benefiting from the hierarchical structure and ultrathin size, when the silicon-based nanosheets are employed as anodes they exhibit a charge (delithiation) capacity of 800 mAh/g after 50 cycles with a maximum coulombic efficiency of 99.4% and good rate performance (647 mAh/g at 1 A/g). This work demonstrates a novel method for preparing nanosheets not only for lithium ion batteries but also having various potential applications in other fields, such as catalysts, electronics and photonics.

Electrochemical behavior and surface structural change of LiMn2O4 charged to 5.1 V

Charging a spinel LiMn2O4 cathode material to high voltage (>4.3 V) is a convenient way to obtain more lithium ions for formation of an anodic solid-electrolyte-interface in a full cell. In this work, a LiMn2O4 spinel cathode material was charged to 5.1 V for only one cycle during normal cycling (3–4.3 V) to study the impact of high voltage on the electrochemical performance and structure. The electrochemical performance showed that more lithium ions were de-intercalated from the cathode structure during cycling between 3 and 5.1 V. However, even upon cycling to high voltage for only one cycle, the surface of the cathode demonstrated a drastic change in the atomic-level structure. Via an advanced scanning transmission electron microscopy (STEM) technique, the formation of a layered-like phase was directly observed on the surface of spinel LiMn2O4 charged to high voltage, implying the instability of the spinel structure at high charge voltage. This observation contradicts to the conventional wisdom that the spinel structure is more stable than the layered structure during lithium intercalation. X-ray photoelectron spectroscopy (XPS) results showed a small amount of manganese with lower oxidation states after being charged to 5.1 V, suggesting an accelerated speed of manganese dissolution.

Understanding the effects of surface reconstruction on the electrochemical cycling performance of the spinel LiNi0.5Mn1.5O4 cathode material at elevated temperatures

Detailed investigation of the influence of surface modification using a typical oxide (TiO2) on the electrochemical cycling performance of LiNi0.5Mn1.5O4 at room temperature (25 °C) and elevated temperature (55 °C) is reported. This spinel cathode material is commonly surface-modified with various metal oxides to improve its electrochemical cycling performance in lithium ion batteries. However, the underlying mechanisms of such a treatment, with respect to the surface crystal structure and chemistry evolution, have remained unclear. Bare-LiNi0.5Mn1.5O4 and TiO2-modified LiNi0.5Mn1.5O4 both show excellent cycling performance, i.e. almost no capacity retention and ∼99% coulombic efficiency for 150 cycles, at room temperature. However, at 55 °C the latter shows significantly better electrochemical cycling performance, with 93% capacity retention and ∼96% coulombic efficiency for 100 cycles, than the former with 70% capacity retention and ∼93% coulombic efficiency. Via advanced electron microscopy techniques, we observed that titanium ions migrated into the surface region of LiNi0.5Mn1.5O4 during the surface modification process at high temperature and reconstructed the (1–3 nm) surface spinel structure into a rocksalt-like structure and the subsurface (several nanometers) into a pseudo-rocksalt-like structure. The reconstruction of the surface and subsurface of the LiNi0.5Mn1.5O4 spinel cathode material mitigates not only the migration of Mn ions from the bulk into the electrolyte but also the formation of a solid state electrolyte interface, which plays a critical role in the improvement of electrochemical cycling performance at elevated temperatures.

Novel 1.5 V anode materials, ATiOPO4 (A = NH4, K, Na), for room-temperature sodium-ion batteries

Due to the abundance of sodium in nature, sodium-ion batteries (SIBs) have attracted widespread attention. Numerous intercalated cathode materials have already been reported, but fewer intercalated anode materials are known. Among these materials, most anodes suffer from low coulombic efficiency and the dendritic growth of sodium due to the lower sodiated voltages (below 1.0 V). To improve the safety performance of batteries, exploring new anode materials which have higher sodiated voltage above 1.0 V is very important. Herein, a series of novel intercalated anode materials, ATiOPO4 (A = NH4, K, Na), is introduced for SIBs at the first time. Preparation of NaTiOPO4 by a traditional solid-state reaction is difficult. So we first synthesized NH4TiOPO4 (NTP) by a simple hydrothermal reaction, KTiOPO4 (KTP) and NaTiOPO4 (NaTP) were each prepared by ion exchange with the respective nitrate. These samples were investigated by electrochemical discharge/charge which showed average sodiated voltages of 1.45 V (NTP), 1.4 V (KTP) and 1.5 V (NaTP); respectively. In situ XRD results indicated that a two-phase reaction mechanism accompanies electrochemical Na insertion/extraction in NaTP. These anode materials are potential candidates for developing SEI-free and high safety SIBs.

Understanding Surface Structural Stabilization of the High-Temperature and High-Voltage Cycling Performance of Al3+-Modified LiMn2O4 Cathode Material

Stabilization of the atomic-level surface structure of LiMn2O4 with Al3+ ions is shown to be significant in the improvement of cycling performance, particularly at a high temperature (55 °C) and high voltage (5.1 V). Detailed analysis by X-ray photoelectron spectroscopy, secondary ion mass spectrometry, scanning transmission electron microscopy-energy-dispersive X-ray spectroscopy, etc. reveals that Al3+ ions diffuse into the spinel to form a layered Li(Alx,Mny)O2 structure in the outmost surface where Al3+ concentration is the highest. Other Al3+ ions diffuse into the 8a sites of spinel to form a (Mn3-xAlx)O4 structure and the 16d sites of spinel to form Li(Mn2-xAlx)O4. These complicated surface structures, in particular the layered Li(Alx,Mny)O2, are present at the surface throughout cycling and effectively stabilize the surface structure by preventing dissolution of Mn ions and mitigating cathode-electrolyte reactions. With the Al3+ ions surface modification, a stable cycle performance (∼78% capacity retention after 150 cycles) and high Coulombic efficiency (∼99%) are achieved at 55 °C. More surprisingly, the surface-stabilized LiMn2O4 can be cycled up to 5.1 V without significant degradation, in contrast to the fast capacity degradation found in the unmodified case. Our findings demonstrate the critical role of ions coated on the surface in modifying the structural evolution of the surface of spinel electrode particles and thus will stimulate future efforts to optimize the surface properties of battery electrodes.

Application of Li2S to compensate for loss of active lithium in a Si–C anode

Mixed silicon and carbon (Si–C) materials with high capacity are ideal candidates for the substitution of graphite or other carbon anodes in lithium-ion batteries. However, the low coulombic efficiency of the Si–C anode in the first cycle due to the formation of a solid electrolyte interphase and the consumption of active lithium have hindered its commercial applications. Here, we report using Li2S as a prelithiation material to compensate for the loss of active lithium in the first cycle and, consequently, to enhance the specific energy of lithium-ion batteries. The Si–C anode has an initial discharge specific capacity of ∼738 mA h g−1 and a charge specific capacity of ∼638 mA h g−1. The prelithiation material with a core–shell structure is prepared by mixing Li2S, Ketjenblack (KB) and poly(vinylpyrrolidone) (PVP) in anhydrous ethanol, which shows a high irreversible capacity of ∼1084 mA h g−1. The effect of the compensation of lost active lithium is verified via a LiFePO4 (Li2S)/Si–C full cell, which exhibits not only a high specific capacity but also a stable cycling performance. The specific energy of the LiFePO4 (Li2S)/Si–C full cell shows a remarkable increase compared to the LiFePO4/Si–C full cell, exhibiting ∼13.4%, ∼26.7%, ∼65.0% and ∼110.2% more specific energy after the 1st, 10th, 100th and 200th cycle, respectively.