Synthesis And Modification Of Li-Ni-Mn-O As Cathode Materials For Li-ion Batteries

The research and development of electrode materials, especially low-cost,high-performance electrode materials are important factors constraining thedevelopment of battery industry, although the specific volume capacity, the averageoperating voltage, operating voltage range, cycle life, self-discharge and other aspectsof lithium-ion battery are better than other rechargeable batteries. Up to now, LiCoO₂is the main material used as commercial lithium-ion battery cathode. However,Li1-XCoO₂ has some disadvantages such as the cost, toxicity and safety. So mucheffort needs to be made to develop alternative lithium-insertion electrodes. Layeredlithium nickel manganese oxides are popular materials because of their low cost,non-toxic and safety performance. Series of lithium nickel manganese oxides arebecoming substitutes for lithium cobalt oxide. LiNi_0.5Mn_0.5O₂ is one of the mostattractive materials, not only because of its high theoretical capacity of 280 mAh g^-1,but also now reaching 200 mAh g^-1 at a low current discharge. However,LiNi_0.5Mn_0.5O₂ synthesized previously often contained impurity phase Li₂MnO₃. Itwas difficult to synthesize pure phase materials. Now a few of ways can producepure-phase LiNi_0.5Mn_0.5O₂, such as ion exchange, double hydroxide precipitation,ultrasonic spray thermal decomposition and so on. It is either too complex tosynthesize a large number of products with these methods or to consume largeamounts of reagents. In addition, because Mn²⁺ can be easily oxidized in the air, itneeds to be operated in the inert gas. Even so, the Li/Ni mixed degree of thesynthesized LiNi_0.5Mn_0.5O₂ remained serious, with fast capacity fading during cycling.Therefore, it is necessary to study new synthetic methods and make a study in depthof these methods to improve the durability of the materials.The first chapter reviews the progress of the lithium ion battery cathode materialsand the status of LiNi_0.5Mn_0.5O₂ cathode material. The topics meaning of this paperand the related to scientific issues needed to be addressed are also presented.In the second chapter, there are some shortcomings such as serious degree ofLi/Ni mixing with the traditional solid state method of high temperature sintering. Inorder to relieve these disadvantages, we prepared high-performanced LiNi_0.5Mn_0.5O₂with the Ni-Mn-O solid solution as precursors. This method is simple, and no complicated conditions. In the precursor, Ni and Mn have been homogeneouslymixied at the atomic level, so the degree of Li/Ni mixing is reduced in a small extentin the obtained LiNi_0.5Mn_0.5O₂ . And the reaction conditions were optimized based onthe reaction mechanism of this method. It was found that the LiNi_0.5Mn_0.5O₂ obtainedwith heating rate of 5oC/min calcined at 850oC for 12h was of the largest intensityratio of I₀₀₃/I₁₀₄ and the most obvious splitting of (006)/(102) and (108)/(110)diffraction peaks. The structure refinement showed the sample under this conditions isof the smallest degree of Li/Ni cation mixing. XPS and XAFS analyses indicate thatthe valence states of nickel and manganese in the as-synthesized product under theconditions are +2, +4, respectively. In addition, under these conditions, the samplesynthesized has the initial discharge capacity of 199.8 mAh g^-1 under the small currentdischarge of 20 mA g^-1. The capacity remained 185 mAh g^-1 after 140 cycle, with only6.7% of the initial capacity fading. Moreover, with the large current discharge of 6C,the capacity of 118.8 mAh g^-1 has remained. So the product has a strong competitiveability with the discharge capacity, cycle life, high-current discharge capacity.In the third chapter, LiNi_xMn_1-xO₂ with different Ni/Mn ratio were synthesizedused Ni-Mn-O solid solution with different ratio of Ni/Mn as precursors. In theprecursor, except the precursor with nickel content of 0.2 contains impurity, the rest ofprecursors are pure phase with spinel structure. The LiNi_xMn_1-xO₂ with differentnickel contents are ofα-NaFeO₂ layered structure. With the increase of nickel content,the quality of the hexagonal firstly becomes better and then worse. Similarly, theintensity ratio of I₀₀₃/I₁₀₄ increased firstly and then decreased. Rietveld refinementresults show that, with the nickel content increased, the degree of Li/Ni cation mixingand the crystal structure parameters are increased. In addition, XPS and XAFSanalyses showed that the valence states of nickel and manganese also varies with thenickel content. When nickel content is lower, the compounds contains Mn³⁺. Whennickel content is higher, compounds contains Ni³⁺. Based on the variation of valencestates and structures, the compounds can be written asaLiNiO₂-bLiNi_0.5Mn_0.5O₂-cLiMnO₂. The electrochemistry analysis shows that whenthe nickel content is smaller, Mn³⁺ of compounds provides no electrochemical activity.Therefore, the LiMnO₂ in the form of the compounds doed not have any capacity.When the nickel content is higher, Ni³⁺ of the compounds provides some capacitybecause LiNi_0.5Mn_0.5O₂ has the maximum capacity, LiNi_0.7Mn_0.3O₂ second,LiNi_0.3Mn_0.7O₂ smallest. In the fourth chapter, different lithium content of LixNi_0.5Mn_0.5O₂ weresynthesized with the precursor of solid solution oxide. By studying their XRDpatterns, it was found that with all the lithium content from 0.5 to 1.5, compounds areofα-NaFeO₂ layered structure with no impurity except a small amount of precursorcompounds are mixed in the material with the lithium content of 0.5. With the lithiumcontent increases, the quality of the hexagonal changed from better to worse, thedegrees of Li/Ni mixing firstly increased and then decreased. XAFS study shows thatwith the lithium content increasing, the valence states of nickel in the compoundsdecreased and then remained unchanged, while the valence states of Mn has oneinflection point at x=1.0. These point were also supported by the soft X-ray analysisof manganese and oxygen. Finally, the electrochemical properties of compounds withdifferent lithium content were studied. And it was found that the electrochemistryperformances of the cathode materials with higher lithium ion concentration were notbetter, but were related with the degree of Li/Ni cation mixing. The electrochemicalproperties with lithium content of 1.5 is not as good as the one in which lithiumcontent is 1.0, and cycling performance is also not as good.Chapter V continues using this method of solid solution as the precursor to studythe modification with cobalt doped. In the precursor, except the precursor of nickel,cobalt and manganese in the ratio of 1:1:1 has impurity of CoO, other precursors arepure phase with spinel structure of NiMn₂O₄. Nickel, cobalt and manganesesufficiently mixed at atomic level. LiNi_0.5-xMn_0.5-xCo₂xO₂ of various cobalt content areofα-NaFeO₂ layered structure. With the increase of cobalt content, structureparameters and the degree of Li/Ni mixing were reduced. XPS analysis shows that thevalence states of nickel, cobalt and manganese were predominantly +2, +3 and +4,respectively, with traces of the Ni³⁺, Co²⁺ and Mn³⁺ because of the electron transition.Electrochemical analysis shows that, LiNi_0.49Mn_0.49Co_0.02O₂ has the bestelectrochemical properties in the 2.5-4.4V with 20 mA g^-1. The capacity retention is86.1% after 50 cycles with the charge and discharge rate of 20 mA g^-1. And theelectrochemical performance of LiNi_0.34Mn_0.33Co_0.33 was studied as a priority. Thedischarge capacity reservations is only 63 mAh g^-1 with the big discharge rate of 1000mA g^-1. When the discharge rate returned to 20 mA g^-1, the capacity re-changed backto 157 mAh g^-1, which was related to the impurity of CoO in the precursor.For the inconsistencies of existing literature on the effects of Al³⁺ doped, ChapterVI continues with the method with the precursors of Ni - Mn - Al-O solid solution to prepare LiNi_0.5-xMn_0.5-xAl_2xO₂ battery materials with different aluminum contents.Similar to the solid solution as mentioned above, Ni - Mn - Al-O solid solutions withdifferent aluminum contents are also of spinel structure, in which nickel, manganeseand aluminum sufficiently mixed in atomic level. All compounds ofLiNi_0.5-xMn_0.5-xAl_2xO₂ with different aluminum contents are of layeredα-NaFeO₂structure and the conditions and processes of preparation were optimized. Structuralrefinement with synthesized LiNi_0.5-xMn_0.5-xAl_2xO₂ in optimized conditions revealedthat the values of c/a increased, degree of Li/Ni cation mixing decrease, but theamount of Ni²⁺ in 3b location reduced with the increase of aluminum content. XPSanalysis shows that for the aluminum doped materials, the valence states of nickel andmanganese are mainly +2 and +4, respectively. The degree of electronic exchangebetween Ni²⁺ and Mn⁴⁺ are lower than that of aluminum undoped, so the amount ofNi³⁺ and Mn³⁺ reduced, but there was no direct relation with the amount of aluminum.Electrochemical performance test results show that, the capacity and cycleperformance of materials can greatly increase after aluminum doped and bothoxidation potential and reduction potential are slightly higher. Among the compoundswith different aluminum content, LiNi_0.475Mn_0.475Al_0.05O₂ has the best electrochemicalproperties. With the discharge current rate of 20mA g^-1, the initial capacity is 206mAhg^-1, and still 96% of initial capacity maintained after 30 cycles, indicating a goodcycling performance. In addition, LiNi_0.425Mn_0.425Al_0.15O₂ has the best cyclingperformance and lower redox potential. 98% of the initial discharge capacity retainedafter 30 cycles. But the capacity is not high because of the less amount of the Ni²⁺ in3b location.Chapter VII summarized the paper's innovations and weaknesses briefly, and thecathode material for future follow-up research are suggested.