Studies On Preparation And Electrochemical Performance Of Anode Materials For Low-temperature Lithium Ion Batteries

Desirable low-temperature performance is one of the important improving aspectsfor the application range of lithium ion batteries. Most current commercial lithium ionbatteries can work only at temperature above -20℃because of higher melting pointof exclusively used EC-based electrolyte. Therefore, PC-based electrolytes arenecessary for low-temperature lithium ion batteries because of their low melting point.However, lithium ion cannot intercalate into graphite reversibly in PC-basedelectrolytes because of ceaseless decomposition of PC at the surface of graphite. Theexisting improving methods for low-temperature performance based on EC-basedelectrolytes are very effective due to the high melting point of EC. The additives forPC and new lithium salts are either expensive, or unstable in electrochemistry.Modifications of anode materials are mostly considered for EC-based electrolytes.Even if they are considered for PC-based electrolytes, the percentage of PC is oftenlower than 30%. Thus the advantage of PC cannot be markedly exhibited.Based on the modification of anode materials, a layer of metal, amorphous carbonor oxide was coated on graphite by various coating methods in this dissertation. Thecoating layer concealed the active sites on the surface of graphite so that it avoided thedecomposition of PC. Meanwhile, the coating effects were investigated by XRD,SEM, CV, EIS and galvanostatic discharge-charge.A layer of Ag and Cu was firstly coated on synthesized graphite called CMSrespectively by electroless plating method. The results from SEM show that Cucoating is more complete and compact although Cu and Ag coating are both quitecomplete. No impurity is introduced by Ag coating from XRD. A little amount ofoxides was detected in Cu-coated composite. Results from galvanostaticdischarge-charge indicate that Ag and Cu coating suppressed the decomposition of PCeffectively and obtained desirable electrochemical performance. Only completecoating layer on CMS can effectively suppress the decomposition of PC. During thefirst discharge-charge of Ag-coated CMS, Ag is involved in the electrochemicalreaction, which is proved by CV. Cu is not involved in the electrochemical reactionfor Cu-coated CMS. The reversible capacity of the first cycle for Cu-coated CMSreaches 350 mAh/g in PC-based electrolyte, which is higher than that for Ag-coatedCMS. The irreversible capacity for Cu-coated CMS is lower than that for Ag-coatedCMS. The cycleability of Cu-coated CMS is better than that of Ag-coated CMS. Result from CV shows that the Cu coating layer acts as a part of SEI film to a certainextent. The measurement of EIS shows that the charge-transfer resistance decreasesand the diffusion coefficient of lithium ions increases after Ag and Cu coating. The Agand Cu coating layer keeps the active sites on the surface of CMS from direct contactwith electrolyte, and therefore greatly suppressed the decomposition of PC on thesurface of graphite and obtained desirable electrochemical performance in electrolytecontaining 50 vol.% PC.Due to the marked suppressing effect of metal coating on decomposition of PC, itmaybe plays the similar role to coat CMS electrode foil directly by a layer of metal. Alayer of Ag and Cu was coated directly on CMS electrode foil by electroless plating. Anew activating method was developed to electroless plate nonmetal. The results fromSEM, XRD and EDX show that Ag and Cu was coated uniformly and completely onthe surface of CMS electrode foil, and the coating percentage is very small. Resultsfrom galvanostatic discharge-charge indicate that Ag and Cu coating significantlysuppressed PC decomposition and obtained desirable electrochemical performance inPC-based electrolyte. The effect of Cu coating is superior to Ag coating, which is alsoproved by CV. The results of EIS show that the charge-transfer resistance decreasesand the diffusion coefficient of lithium ions increases after Ag and Cu coating. The Agand Cu coating layer keeps the active sites on the surface of CMS from direct contactwith electrolyte, and therefore greatly suppressed the decomposition of PC andobtained desirable electrochemical performance in electrolyte containing 50 vol:% PC.Compared with Ag and Cu coating on CMS particles, this method has a lower cost.Amorphous carbon was considered to coat CMS due to its good compatibility withPC-based electrolytes. A layer of PAN and sucrose was coated on graphite byemulsion polymerization and dip coating respectively, and then pyrolyzed intoamorphous carbon on CMS at low temperature. Sucrose pyrolytic carbon coating ismore complete and compact from SEM. Results from XRD show that the intensity ofpeak for graphite (002) decreases and interlayer distance slightly increases afteramorphous carbon coating. Results from galvanostatic discharge-charge indicate thatsucrose pyrolytic carbon coating suppressed PC decomposition more effectively,while PAN pyrolytic carbon-coated CMS can only be cycled in electrolyte containing35 vol.% PC. Result of CV also proves the suppression of amorphous carbon coatingon PC decomposition though there is still slight decomposition of PC. The results ofEIS show that the charge-transfer resistance decreases and the diffusion coefficient of lithium ions increases after amorphous carbon coating. The amorphous carbon coatinglayer keeps the active sites on the surface of CMS from direct contact with electrolyte,and therefore suppressed the decomposition of PC greatly and obtained desirableelectrochemical performance in PC-based electrolyte.Inorganic oxides can also be coated on graphite. A layer of nano TiO₂ was coatedon CMS by mechano-thermal coating process. Result of XRD shows that theinterlayer distance of graphite increases slightly despite of no crystal changes of TiO₂.TiO₂ coating on CMS is quite uniform and complete from SEM. Results fromgalvanostatic discharge-charge indicate that TiO₂ coating significantly suppressed PCdecomposition mainly because coated TiO₂ particle keeps the active sites on thesurface of CMS from direct contact with electrolyte. Due to the limit of cut-offvoltage, TiO₂ coating layer basically keeps inert after the first cycle, which results inmore stable cycleability of TiO₂-coated CMS in PC-based electrolyte than that of theoriginal CMS in EC-based electrolyte. Result of CV also proves the suppression ofTiO₂ coating on PC decomposition. The results of EIS show that both charge-transferresistance and the diffusion coefficient of lithium ions increases after TiO₂ coating.The above coating methods suppressed the decomposition of PC on the surface ofCMS to different extent. By and large, the electrochemical performance of coatedcomposite in PC-based electrolyte achieves the same level to that of the original CMSin EC-based electrolyte. TiO₂ (from the second cycle) and Cu coating layer is inert forlithium ion and acts as a buffer for the volumetric changes of graphite during cycles.Thus cycleability of graphite can be improved by inert materials coating. Amorphouscarbon coating needs to be improved since it doesn't suppress PC decompositionperfectly. Cu coating is superior to other materials coating whether coating on CMSparticles or on CMS electrode foil. Therefore Cu coating is a perfect method formodification of graphite in view of practice. It can be concluded that thelow-temperature operation limit of lithium ion batteries will be greatly improved bythe coating methods mentioned in this dissertation together with PC-based electrolytes.It will be very promising for lithium ion batteries with the mentioned composite anodematerials to achieve desirable electrochemical performance below -60℃.