Facile one-pot synthesis of Ge/TiO2 nanocomposite structures with improved electrochemical performance

Germanium Nanowires via Molten-Salt Electrolysis for Lithium Battery Anode

Germanium (Ge)-based materials can serve as promising anode candidates for high-energy lithium-ion batteries (LIBs). However, the rapid capacity decay caused by huge volume expansion severely retards their application. Herein, we report a facile and controllable synthesis of Ge nanowire anode materials through molten-salt electrolysis. The optimal Ge nanowires can deliver a capacity of 1058.9 mAh g^-1 at 300 mA g^-1 and a capacity above 602.5 mAh g^-1 at 3000 mA g^-1 for 900 cycles. By in situ transmission electron microscopy and in situ X-ray diffraction, the multiple-step phase transformation and good structural reversibility of the Ge nanowires during charge/discharge are elucidated. When coupled with a lithium-rich Li_1.2Mn_0.567Ni_0.167Co_0.067O₂ cathode in a full battery, the Ge nanowire anode leads to a relatively stable capacity with a retention of 84.5% over 100 cycles. This research highlights the significance of molten-salt electrolysis for the synthesis of alloy-type anode materials toward high-energy LIBs.

A Synergistic Effect of Na+ and Al3+ Dual Doping on Electrochemical Performance and Structural Stability of LiNi0.88Co0.08Mn0.04O2 Cathodes for Li-Ion Batteries

The synergistic effect of Na⁺/Al³⁺ dual doping is investigated to improve the structural stability and electrochemical performance of LiNi_0.88Co_0.08Mn_0.04O₂ cathodes for Li-ion batteries. Rietveld refinement and density functional theory calculations confirm that Na⁺/Al³⁺ dual doping changes the lattice parameters of LiNi_0.88Co_0.08Mn_0.04O₂. The changes in the lattice parameters and degree of cation mixing can be alleviated by maintaining the thickness of the LiO₆ slab because the energy of Al-O bonds is higher than that of transition metal (TM)-O bonds. Moreover, Na is an abundant and inexpensive metal, and unlike Al³⁺, Na⁺ can be doped into the Li slab. The ionic radius of Na⁺ (1.02 Å) is larger than that of Li⁺ (0.76 Å); therefore, when Na⁺ is inserted into Li sites, the Li slab expands, indicating that Na⁺ serves as a pillar ion for the Li diffusion pathway. Upon dual doping of the Li and TM sites of Ni-rich Ni_0.88Co_0.08Mn_0.04O₂ (NCM) with Na⁺ and Al³⁺, respectively, the lattice structure of the obtained NNCMA is more ideal than those of bare NCM and Li⁺- and Na⁺-doped NCM (NNCM and NCMA, respectively). This suggests that NNCMA with an ideal lattice structure presents several advantages, namely, excellent structural stability, a low degree of cation mixing, and favorable Li-ion diffusion. Consequently, the rate capability of NNCMA (83.67%, 3 C/0.2 C), which presents favorable Li-ion diffusion because of the expanded Li sites, is higher than those of bare NCM (78.68%), NNCM (81.15%), and NCMA (83.18%). The Rietveld refinement, differential capacity analysis, and galvanostatic intermittent titration technique results indicate that NNCMA exhibits low polarization, favorable Li-ion diffusion, and a low degree of cation mixing; moreover, its phase transition is hindered. Consequently, NNCMA demonstrates a higher capacity retention (84%) than bare NCM (79%), NNCM (82%), and NCMA (82%) after 50 cycles at 1 C. This study provides insight into the fabrication of Ni-rich NCMs with excellent electrochemical performance.

Sn modified nanoporous Ge for improved lithium storage performance

Although high-capacity germanium (Ge) has been regarded as the promising anode material for lithium ion batteries (LIBs), its actual performance is far from expectation because of low electrical conductivity and rapid capacity decay during cycling. In this work, Sn modified nanoporous Ge materials with different Ge/Sn atomic ratios in precursors were synthesized by a simple melt-spinning and dealloying strategy. As the anodes of LIBs, Sn modified nanoporous Ge materials display improved cycling stability compared with Sn-free nanoporous Ge, revealing a potential role of Sn in improving electrochemical properties of Ge-based anodes. In particular, Sn modified nanoporous Ge with Ge/Sn atomic ratio of 3:1 presents the best Li storage performance among measured electrodes, delivering a reversible capacity of 974 mA h g^-1 after 500 cycles at 200 mA g^-1. It is found that the introduction of appropriate amount of Sn can not only regulate the nanoporous structure of Ge to better alleviate volume expansion, but also improves the conductivity and activity of the electrode material. This improvement is demonstrated by density functional theory calculations. The study uncovers a route to improve Li storage properties by rationally modify Ge-based anodes with Sn, which may facilitate the development of high-performance LIBs.

Fluorine-Doped LiNi0.8Mn0.1Co0.1O2 Cathode for High-Performance Lithium-Ion Batteries

For advanced lithium-ion batteries, LiNixCoyMnzO2 (x + y + z = 1) (NCM) cathode materials containing a high nickel content have been attractive because of their high capacity. However, to solve severe problems such as cation mixing, oxygen evolution, and transition metal dissolution in LiNi0.8Co0.1Mn0.1O2 cathodes, in this study, F-doped LiNi0.8Co0.1Mn0.1O2 (NCMF) was synthesized by solid-state reaction of a NCM and ammonium fluoride, followed by heating process. From X-ray diffraction analysis and X-ray photoelectron spectroscopy, the oxygen in NCM can be replaced by F− ions to produce the F-doped NCM structure. The substitution of oxygen with F− ions may produce relatively strong bonds between the transition metal and F and increase the c lattice parameter of the structure. The NCMF cathode exhibits better electrochemical performance and stability in half- and full-cell tests compared to the NCM cathode.

An artificial sea urchin with hollow spines: improved mechanical and electrochemical stability in high-capacity Li-Ge batteries

Metallic germanium (Ge) as the anode can deliver a high specific capacity and high rate capability in lithium ion batteries. However, the large volume expansion largely restrains its further application. Herein, we constructed a three-dimensional sea urchin structure consisting of double layered Ge/TiO₂ nanotubes as the spines via a ZnO template-removing method, which displays a capacity as high as 1060 mA h g^-1 over 130 cycles. The robust, hollow oxide backbone serves as a strong support to accommodate the morphological change of Ge while the enhanced electron-transfer kinetics is attributed to the Ge content and the intimate contact between Ge and TiO₂ during charging/discharging, which were confirmed using in situ transmission electronic microscopy observations and first-principles simulations. In addition, a high capacity retention of batteries using this hybrid composite as the anode was also achieved at low temperature.

Porous SnO2 nanostructure with a high specific surface area for improved electrochemical performance

Tin oxide (SnO₂) has been attractive as an alternative to carbon-based anode materials because of its fairly high theoretical capacity during cycling. However, SnO₂ has critical drawbacks, such as poor cycle stability caused by a large volumetric variation during the alloying/de-alloying reaction and low capacity at a high current density due to its low electrical conductivity. In this study, we synthesized a porous SnO₂ nanostructure (n-SnO₂) that has a high specific surface area as an anode active material using the Adams fusion method. From the Brunauer–Emmett–Teller analysis and transmission electron microscopy, the as-prepared SnO₂ sample was found to have a mesoporous structure with a fairly high surface area of 122 m² g⁻¹ consisting of highly-crystalline nanoparticles with an average particle size of 5.5 nm. Compared to a commercial SnO₂, n-SnO₂ showed significantly improved electrochemical performance because of its increased specific surface area and short Li⁺ ion pathway. Furthermore, during 50 cycles at a high current density of 800 mA g⁻¹, n-SnO₂ exhibited a high initial capacity of 1024 mA h g⁻¹ and enhanced retention of 53.6% compared to c-SnO₂ (496 mA h g⁻¹ and 23.5%).

A porous SnO₂ nanostructure as an anode active material showed significantly improved electrochemical performance.