Lithium-ion batteries have become an ideal energy source for active
development in the 21st century due to their high operating voltage, high
specific energy, no memory effect, no pollution, small self-discharge, and long
cycle life. The key to the development of lithium-ion batteries is the
development of electrode materials. At present, compared with the negative
electrode materials of lithium-ion batteries, the development of positive
electrode materials is slow, which has become the main factor inhibiting the
improvement of battery specific energy. The cathode materials mainly focus on:
cobalt, manganese, nickel oxide lithium salts and olivine phosphates and their
dopant series materials. Lithium manganate is considered to be one of the most
promising cathode materials for lithium ion batteries due to its advantages of
abundant resources, no pollution, low price, resistance to overcharge and
overdischarge, and good thermal stability. Lithium manganate has two structures:
spinel and layered. Due to the poor cycling performance of spinel-type lithium
manganate, especially the serious capacity attenuation at high temperatures, it
has not been widely used yet. Layered LiMnO2 has the advantages of non-toxic,
safe, and high theoretical capacity (about 285mAh/g). It has become a hot spot
in current research and has very broad application prospects in fields such as
electric vehicles and space technology. my country's manganese reserves are very
abundant, and the application of manganese oxide lithium cathode materials can
greatly reduce the cost of current commercial lithium-ion batteries.
Accelerating the research on LiMnO2 cathode materials is of great significance
to my country's lithium-ion battery industry. 1. Research progress of layered
lithium manganate 1.1 Structure of layered lithium manganate The ideal layered
LiMnO2 structure belongs to the trigonal crystal system. O atoms are arranged in
a slightly twisted cubic tightly packed arrangement. Mn atoms are located in the
octahedral layer, while Li atoms in adjacent octahedral layers. But LiMnO2 is
actually a homogeneous polycrystalline compound with two structures:
orthorhombic and monoclinic. Orthorhombic LiMnO2 has a layered rock salt
structure, and monoclinic LiMnO2 has an α-NaFeO2 type layered structure. This
structure is not thermodynamically stable. phase, which is difficult to
synthesize under equilibrium conditions. The electrochemical performance of
layered LiMnO2 is closely related to its synthesis method. The material particle
size, morphology, specific surface area, crystallinity and lattice defects
obtained by different synthesis methods are significantly different. These
factors affect the "embedding and extraction" of lithium ions. Reaction has a
decisive influence. 1.2 Synthesis of layered lithium manganate Layered LiMnO2
can be synthesized using a variety of techniques, such as high-temperature
solid-phase synthesis, sol-gel synthesis, template method, hydrothermal
synthesis, co-precipitation synthesis, etc. (1) High-temperature solid-phase
synthesis method High-temperature solid-phase synthesis of LiMnO2, that is,
lithium salts (such as Li2CO3, LiNO3, LiOH·H2O, etc.) and manganese salts [such
as Mn(NO3)2, MnCO3, etc.] or manganese oxides (such as Mn3O4 , MnO2, etc.) are
ground and mixed in a certain way, and then sintered at high temperature for a
period of time in a suitable furnace atmosphere (Ar or N2). The raw materials
undergo solid phase reaction directly at high temperature to obtain the required
sample. Li Yibing et al. roasted MnO2 to obtain Mn2O3, and then mixed it with
LiOH·H2O and roasted it to obtain synthetic o-LiMnO2 powder. The solid-phase
reaction method has a simple process and is suitable for industrial mass
production. However, the contact between the reactants is uneven and the
reaction is insufficient. Therefore, soft chemical methods are mainly used in
current experiments to prepare materials with excellent electrochemical
properties. The disadvantages are: serious loss of lithium, difficulty in
controlling the stoichiometric ratio, difficulty in controlling the particle
size of the synthesized product, uneven distribution, and irregular morphology.
(2) Sol-gel method Sathiya et al. mixed metal alkoxides LiNO3, Mn(NO3)2,
Co(NO3)2, and NiNO3 in a solution according to a certain stoichiometric ratio,
used citric acid as a chelating agent, and added ethylenediamine The pH value of
the solution is adjusted, and a sol is formed through the reaction. The mixture
is then stirred and undergoes dehydration and polycondensation reaction to form
a gel. The gel is vacuum dried and solidified, and then sintered to form the
sample to be prepared. This method has a lower reaction temperature and shorter
reaction time, but the raw materials are more expensive. (3) Co-precipitation
method Co-precipitation method is to add a precipitant to a mixed solution
composed of soluble salts with different chemical components in a solution state
to form insoluble ultrafine particles, that is, precursor precipitates, and then
precipitate these A method of drying or calcining materials to obtain
corresponding ultrafine particles. Fan et al. added NH4HCO3 as a precipitant to
the MnCl2 solution to prepare S-MnCO3. MnCO3 is calcined in air to obtain Mn2O3,
which is mixed with LiOH·H2O and calcined under argon gas protection to obtain
So-LiMnO2. (4) Melting method Wang Chengwei and others evenly mixed LiCl and
LiNiO3 (the mass ratio of the two is 1:3) into a porcelain crucible, raised the
temperature to 300°C in a muffle furnace, and added NaMnO2 powder to the
crucible after the mixture was melted. Stir evenly, treat at constant
temperature for 4 hours, then cool and wash with deionized water. The sample is
vacuum dried at 120°C to obtain LiMnO2. (5) Hydrothermal synthesis is a
synthesis in which substances in the solvent undergo chemical reactions under
certain conditions of temperature (100~1000℃) and pressure (1~100MPa). Zhou et
al. used homemade γ-MnOOH to disperse and dissolve in LiOH·H2O solution in a
high-pressure reactor to obtain o-LiMnO2 with a diameter of 80 to 100 nm. Komaba
et al. added O2 to the Mn(CH3COO)2·4H2O and KOH solutions, stirred them,
filtered out the precipitate to obtain Mn3O4, mixed it with LiOH·H2O aqueous
solution and put it into an autoclave, and reacted to obtain o-LiMnO2.
Hydrothermal synthesis is conducive to the generation of low valence state,
intermediate valence state and special valence state compounds, and can be doped
uniformly. Powders obtained by hydrothermal synthesis generally have high
crystallinity and small crystal defects, and the size, uniformity, shape,
composition, etc. of the powder can be strictly controlled. (6) Ion exchange
method Ion exchange method is a method that uses the mutual exchange reaction
between anions or cations in solid ion exchangers and ions in liquid to
separate, purify or prepare new substances. The use of this method to prepare
LiMnO2 is based on the fact that NaMnO2 has a greater affinity for lithium ions
than sodium ions, thereby producing LiMnO2 through an exchange reaction with
lithium ions in the lithium salt solution. Xu Tianjun et al. used ball milling
solid phase reaction to obtain NaMnO2, and then immersed it in a mixed molten
salt of LiNO3 and LiCl to prepare m-LiMnO2. SEM and TEM observation results show
that the particle size of LiMnO2 is 300~500nm, and the synthesized LiMnO2 shows
good electrochemical cycle performance. (7) Template method Zhou Zhenhua et al.
dissolved lithium acetate and manganese acetate in absolute ethanol, weighed a
certain amount of silica gel and soaked it in the solution. It was first
calcined in a muffle furnace at a constant temperature of 300°C for 4 hours, and
then in a nitrogen atmosphere furnace at 700 Calcined at ℃ for 10h. Then
dissolve the silica gel template in 4 mol/L NaOH solution to obtain nano-sized
LiMnO2 powder. After characterization, the MnO2 prepared by the template method
has good crystallinity, high purity, and a particle size of about 30nm. 2.
Doping modification of layered lithium manganate Doping modification is a common
method to stabilize the structure and performance of materials. Doping in
layered lithium manganate introduces a small amount of other metal ions, such as
Co3+, Al3+, Co3+, Ni3+ Equal doping can increase the valence state of Mn,
effectively suppress the Jahn Teller effect, significantly improve the cycling
stability of layered lithium manganese oxide at room temperature, and also
improve the cycling stability at high temperatures. 2.1 Cation doping The
current suppression of Jahn Teller distortion mainly uses cation doping: ①
Incorporating low-valent metal ions to increase the average valence state of
manganese, thereby effectively suppressing the occurrence of the Jaln-Teller
effect; ② Incorporating a larger radius elements, improve the unit cell
parameters of the material, increase the unit cell volume, and promote the
diffusion of lithium ions. 2.1.1 Single atom doping (1) The ionic radius and
octahedral site selection energy of Co-doped Co3+ are similar to those of Mn3+.
After layered LiMnO2 is doped with Co, Co3+ occupies the original Mn3+
octahedral position, causing the Jahn-Teller effect on Mn3+ It has a certain
inhibitory effect, thereby stabilizing the layered structure and improving the
electrochemical properties of the material. Oh et al. used ultrasonic spray
drying to synthesize Li[Ni0.5Mn0.5]1-xCoxO2. When the content of Co3+ increases,
the discharge capacity of the material increases. In LiNi0.5Mn0.5O2, the valence
states of Ni and Mn are each +2. , +4. The addition of Ni2+ increases the
average valence state of Mn and improves the stability of the material. (2) Cr
doping Myung et al. used Cr doping LiMnO2 to obtain LiMn1-xCrxO2. Research
points out that the doping of Cr helps to improve the electrochemical
performance. The less the amount of Cr doping, the higher the capacitance of the
material. The ionic radius of Cr3+ is smaller than that of Mn3+, which leads to
the shortening of the Mn-O bond and the reduction of the grain size, which
stabilizes the Mn3+ in the octahedral position to a certain extent, thus
improving the stability. (3) Ni doping Since Ni is difficult to oxidize under
normal conditions, after Ni is introduced into layered LiMnO2, the oxidation
state of Ni is generally 2 valence, Ni2+ occupies the position of Mn3+, and the
valence state of the transition metal is maintained at +3, resulting in Mn The
average valence of Mn3+ increases, reducing the Jahn-Teller effect of Mn3+,
thereby stabilizing the layered structure and improving the structural stability
of the material. Yong et al. synthesized Li[NixLi(1/3-2/3)Mn(2/3-x/3)]O2
(x=0.17, 0.25, 0.33, 0.5). Through comparison, it was found that with the
increase of Ni component Reduced, the voltage platform becomes longer and the
phase structure becomes more complete. Research shows that lower Ni content can
also increase charge-discharge capacity and initial irreversible capacity. He
Pingping used the precipitation method to synthesize LiNixMn1-xO2 and analyzed
its structure. He found that when n(Ni)/n(Mn)<1, as the Ni content increases,
Ni's occupation of Li sites will become more serious. The more Ni occupies Li in
the 3a position, the smaller the amount of Li+ desorption and insertion, which
in turn affects the electrochemical properties of the cathode material. (4) Fe
and Zn doping Suresh et al. used deionization method to prepare LiMn1-xMxO2
(M=Fe, Zn). By doping Fe and Zn, they found that doping Fe and Zn can improve
the cycle performance of the material. Resulting in a decrease in capacitance,
LiMn0.95Zn0.05O2 and LiMn0.95Fe0.05O2 reached maximum capacitances of 180mAh/g
and 193mAh/g respectively, and still had very good capacities after several
cycles. The study found that the decrease in capacity caused by the increase in
Zn component was mainly due to the decrease in electrochemically active Mn3+,
while the decrease in capacity caused by the increase in Fe was due to the
increase in Fe occupying Li positions, causing spinel deformation. Single
element doping is not ideal for improving the performance of LiMnO2. If two or
more component dopings are combined, the overall performance of LiMnO2 can be
comprehensively improved. 2.1.2 Polyatomic doping (1) Ni and Co doping Xiao et
al. used co-precipitation method to dope Ni and Co elements to prepare
LiNi0.9-yMnyCo0.1O2. When y is equal to 0.5, some Mn ions in LiNi0.4Mn0.5Co0.1O2
become Mn3+, but it is not enough to prevent the J-T effect. When y is less than
50%, the initial charge and discharge capacities of LiNi0.4Mn0.5Co0.1O2 are
218mAh/g and 181mAh/g respectively, but the rate performance is slightly worse.
When the Mn content exceeds 50%, the capacity will decrease and the polarization
phenomenon will increase strongly. Zhong et al. [32] used Ni(CH3COO)2?4H2O,
Co(CH3COO)2?4H2O, Mn(CH3COO)2?4H2O as raw materials, and Na2O3 as the
precipitant to prepare LiNi0.6Co0.2Mn0.2O2. LiNi0.6Co0.2Mn0.2O2 is prepared at
different temperatures and calcination times, and the electrochemical properties
have certain differences. LiNi0.6Co0.2Mn0.2O2 has the most uniform particle size
when calcined at 850°C. LiNi0.6Co0.2Mn0 The first discharge capacity of .2O2 is
148mAh/g at 850℃, and the power loss is minimum 8.2% after 30 cycles. It shows
that Ni and Co atom doping can improve the electrochemical performance to a
certain extent. After Co is incorporated, it occupies the octahedral position of
Mn3+, which can stabilize the structure and improve conductivity and thermal
stability. At the same time, the incorporation of Ni can increase the average
valence state of Mn and stabilize the structure. Let each element play its own
different role to jointly improve the electrochemical performance of the
material. (2) Cr and Li doping Tang Aidong et al. used a liquid phase method to
mix manganese, chromium, and lithium reactants at the molecular and atomic
levels, and synthesized Li/Li (CrxLi1/3-x/3Mn2/3-x/ 3)O2. Constant current
charge and discharge tests were performed at 25°C, 2.5~4.5V, and the current
density was 14mA/g. It was found that the maximum discharge capacity of the
material could reach 221mAh/g, and the capacity was 180mAh/g after 10 cycles.
Research has found that the unit cell parameters a and c of the material
increase with the incorporation of Cr. Larger unit cell parameters are conducive
to the drag and insertion of lithium ions in the material, thereby improving the
electrochemical performance of the material. (3) Cr and Ni doped Kim and others
synthesized LiCrxNi0.5-xMn0.5O2 (x=0, 0.05, 0.1) through co-precipitation
method. The experimental results show that the first discharge capacity of
LiNi0.5Mn0.5O2 is 185mAh/g, 15 After 1 cycle, it is 150mAh/g. Due to the effect
of Cr doping, the capacity of Cr0.05Ni0.45Mn0.5O2 is higher than that of
LiCr0.1Ni0.4Mn0.5O2. The less the amount of Cr doping, the higher the initial
capacity, but the degree of capacity attenuation increases with the increase of
Cr. And become more slowly. (4) Ni, Ti-doped Li, etc. are prepared by spray
drying method to prepare LiNi0.5Mn0.5-xTixO2. As the Ti component increases, the
first charge and discharge capacity decreases. When x=0, the first discharge
capacity is 190mAh/ When g, x=0.15, the first discharge capacity is 150mAh/g.
When the Ti component is less than 0.3, it exhibits good cycle performance.
After analysis, as the Ti component increases, the degree of cation mixing also
deepens, and the crystal structure gradually changes from layered to a chaotic
rock salt structure. This phenomenon may be related to the intensification of
cation mixing. 2.2 Anion-doped layered LiMnO2. In addition to doping cations to
improve its electrochemical performance, doping anions will also achieve the
same effect. After layered LiMnO2 is doped with anions, Mn2+ or Mn4+ will be
produced, which will compensate for the charge of Mn3+, interfere with the
magnetic interaction between Mn3+, and thus stabilize the layered structure.
Li0.7MnO2-ySy synthesized by Park et al. using an ion exchange method has a
particle diameter of 100 to 200nm and a first discharge capacity of 170mAh/g at
2 to 4.6V. The material will gradually change to spinel type during cycling, but
the cycle
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