Study Of Layered Titanium Hydrogen Phosphates For Both Li-ion And Na-ion Storage

To resolve current crisis of energy and realize environmental protection,the exploration and utilization of renewable energy resources is an inevitable choice.Rechargeable batteries,as the equipement for energy storage and conversion,play an indispendable role on energy storage and conversion.The progress of battery technologies is promoted by continuously increased demands for clean energy resources.But the breakthrough of battery technologies greatly depends on the development of materials.Therefore,it is an important research objective for material scholars to explore new battery materials.Layered electrode materials have embraced widely attention due to its high capacity,easy of synthesis and tunable structure.Both ofα-andγ-titanium hydrogen phosphate possesses lamellar structure,and their protons within P-OH groups can be exchanged by alien ions（mainly monovalent or divalent cations）.Therefore,there are extensive reports on titanium hydrogen phosphates in these fileds like waste purification,ion adsorption,proton conduction,and catalysis,etc.However,the research of titanium hydrogen phosphates on battery is very rare.Here the electrochemical performance of the A-TiP and G-TiP in both lithium and sodium storage was systematically investigated.Firstly,the electrochemical performance of the bulk G-TiP in both lithium and sodium storage was investigated.For Na-ion battery,the G-TiP electrode delivers a reversible capacity of 161.3 mAh g^-1 under a voltage cutoff of 0.1-2.8 V,and it exhibits a plateau at range of 1.6-2.2 V during sodiation process.The layered structure of G-TiP is well maintained during repeated（de）sodiation.For Li-ion battery,the G-TiP electrode shows no obvious voltage plateau in voltage profiles upon（de）lithiation when discharged to 0.5 V or below.Because of its layered framework is destroyed,the G-TiP electrode shows a capacitive lithium storage process and the capacity faded fast.But when elvelating lower voltage from 0.1 V to 0.8 V or above,the G-TiP electrode delivers a capacity of 109 mAh g^-1,and its discharge/charge curves obviously show a（de）lithiation voltage plateau at 2.2-2.6 V.The mechanism analysis of G-TiP electrode demonstrates that the G-TiP material can both storage Li and Na via a redox reaction and local ion exchange process within a wide voltage range,showing an unique electrochemical reaction process.In addition,though the G-TiP delivers a decreased capacity in both lithium and sodium battery after removal of its interlayer water,the cycling stability of batteries can be improved greatly.Therefore,dehydration of the G-TiP material is in favor of enhancing its structural stability and then improving its cycling performance.Secondly,effects of ion exchange on both strcture and electrochemical performance in sodium storage of A-TiP was studied.The ion exchanged material Li-TiP was synthesed via a facile solution ion-exchange method.On the one hand,the Li-TiP material maintains the structural framework like that of original A-TiP material.Compared to the A-TiP material,the Li-TiP electrode shows no obvious difference on nature of redox.And both the A-TiP and Li-TiP electrode show a plateau at 1.8-2.1 V during sodiation,corresponding to a solid solution reaction;On the other hand,because of slowly kinetics upon（de）sodiation,which results from enhanced steric hindrance due to Li⁺exchange,the Li-TiP electrode delivers a decreased capacity compared to the parent A-TiP electrode.But Li⁺exchange is favorable to keep its structure stability and then get improved cycling performance.For instance,after 100 cycles,the capacity retention is improved from56.8%for the original A-TiP electrode to 96.4%for the Li⁺exchanged Li-TiP electrode.A new one ion storage mechanism is presented by this work via study of layered titanium hydrogen phosphates for both lithium and sodium storage.Therefore,this work provides certain guidance for Li-and Na-batteries in designing and exploring new electrode materials.