Lithium, nickel, manganese mixed oxide compound and electrode comprising the same

Abstract

A compound of the general formula: (i) wherein x has a value greater than 0.06 and equal to or less than 0.4. The compound is also formulated into a positive electrode for use in an electrochemical cell.

Description

REFERENCE TO RELATED APPLICATIONS

This application is a national stage application under 35 U.S.C. 371 of International Application No. PCT/GB2018/053660, filed Dec. 18, 2018, which claims the priority of United Kingdom Application No. 1721172.3, filed Dec. 18, 2017, the entire contents of each of which are incorporated herein by reference.

FIELD OF THE DISCLOSURE

The present invention relates to a set of electroactive cathode compounds. More specifically the present invention relates to a set of high capacity lithium-rich NM compounds.

BACKGROUND OF THE DISCLOSURE

Conventional lithium ion batteries are limited in performance by the capacity of the material used to make the positive electrode (cathode). Lithium rich blends of cathode materials containing blends of nickel manganese cobalt oxide offer a trade-off between safety and energy density. It is understood that charge is stored in the transition metal cations within such cathode materials. It has been suggested that the capacity, and therefore energy density, of cathode materials could be significantly increased if charge could be stored on anions (for example oxygen) reducing the need for such high amounts of heavy transition metal ions. However, a challenge remains to provide a material that can rely on the redox chemistries of both the anions and cations to store charge, and withstand charge/discharge cycles without compromising the safety of the material, or causing undesired redox reactions which would break down the material.

SUMMARY OF THE DISCLOSURE

In a first aspect, the present invention provides a compound of the general formula:

${\mathrm{Li}}_{\left(\frac{4}{3}-\frac{2x}{3}\right)}{\mathrm{Ni}}_x{\mathrm{Mn}}_{\left(\frac{2}{3}-\frac{x}{3}\right)}{O}_2$ wherein x has a value greater than 0.06 and less equal to or less than 0.4.

It has been found that a compound with an improved capacity can be achieved by reducing the amount of excess lithium and increasing the amount of nickel. The particular compound as defined above exhibits a significantly large increase in capacity due to the degree of oxidation of nickel and also the oxidation of the oxide ions within the lattice. Without wishing to be bound by theory, it is understood that the presence of a particular amount of nickel substitution enables oxygen redox activity and thereby improves the electrochemical capacity of the material.

In addition, the compounds of the present invention exhibit improved stability during electrochemical cycling when compared to the transition metal substituted NMC lithium rich materials of the prior art. The evolution of molecular oxygen is ubiquitous with third row lithium-rich materials transition metal oxides where lithium has been exchanged for some of the transition metal ions (Li_1+xM_1−xO₂, where M is Ti, V, Cr, Mn, Fe, Co, Ni, Cu or Zn). These materials generally rely on oxygen redox to improve their charge capacity properties. Homogenous materials can suffer from molecular oxygen escaping from the crystal structure during cycling due to redox of the oxide anion. In turn, this reduces the capacity and useful lifetime of the material. However, the material of the present invention has improved capacity which is maintained over numerous cycles.

It is understood that when the charge imbalance caused by the removal of a lithium ion is balanced by the removal of an electron from the oxygen anion the resulting oxygen anion is unstable which results in undesired redox reactions and the evolution of molecular oxygen gas during charge cycling. Without wishing to be bound by theory, it is understood that the specific nickel content in the material relative to the lithium content avoids under-bonding within the lattice such that each oxygen anion is still bonded to ˜3 cations. A potential solution to this problem might be to encapsulate the cathode layer or part of the cell in a gas impermeable membrane. However, this would add parasitic mass to the cell, thereby reducing the energy density of the resulting battery. However, the chemical approach of the present invention tunes the structure of the lattice using specific amounts of transition metals reduces the generation of oxygen gas from the material without the need to add layers to the cathode material or resulting battery cell.

In examples, x (i.e. the nickel content) is equal to or greater than 0.12. x may be equal to or greater than 0.2. It has been demonstrated that capacity of the material is significantly improved when x is equal to or is greater than 0.12, and is further improve when the value of x is equal to 0.2. In addition x may be equal to or less than 0.4. It is understood that the capacity of the material declines to expected levels above this threshold value of 0.4. It has been demonstrated that improved capacity is achieved when x is 0.3. More specifically, the value of x could be said to be greater than 0.06 and equal to or less than 0.4. More specifically, the value of x could be said to be equal to or greater than 0.12 and equal to or less than 0.4. Materials within this broad range show an improved capacity. In further examples of improved materials within this broad range, the value of x may be greater than 0.06 and equal to or less than 0.12.

In further examples, the range of x may be a greater than 0.2. It has been demonstrated that material with a stoichiometric nickel content of above 0.2 has the surprising benefit of reducing the amount of molecular oxygen gas evolved during a charge/discharge cycle as well as having an improved charge capacity. More particularly, this range may be defined as x being greater than 0.2 and equal to or less than 0.4. Even more particularly this range could be defined as x being equal to or greater than 0.3 equal to or less than 0.4. Most particularly, x has a value of either 0.3 or 0.4.

The compound may be defined as having a layered structure. Typically layered structures have been shown to have the highest energy density. When in the layered form, the material can be further defined using the general formula aLi₂MnO₃. (1−a)LiNi_0.5Mn_0.5O₂ such that a may be less than 0.88, a may also be equal to or greater than 0.2. More particularly, a is equal or greater than 0.2 and less than 0.88. Even more particularly, a is equal or greater than 0.2 and equal to or less than 0.76. Specifically the material may be 0.4Li₂MnO₃.0.6LiNi_0.5Mn_0.5O₂, or the material may be 0.2Li₂MnO₃.0.8LiNi_0.5Mn_0.5O₂. These particular layered structures exhibit improved capacity and a higher degree of stability during a charge/discharge cycle. More specifically, the amount of gas evolved from the layered material during a charge/discharge cycle is reduced.

In a second aspect, the present invention provides an electrode comprising the compound of the first aspect. The electrode may comprise 3 fractions. The first is the compound of the present invention as previously described (in a variety of mass percentages from 60-98%, however, typically 70, 75, 80, 90 and 95%). The second fraction of the electrode comprises electroactive additives such as carbon, for example, Super P® and Carbon black, which comprises 60-80% of the mass fraction remaining excluding the first fraction. The third fraction is typically a polymeric binder such as PVDF, PTFE, NaCMC and NaAlginate. In some case additional fractions may be included and the overall percentages may change. The overall electrochemical performance of the cathode material can be improved by the introduction of electroactive additives, and the structural properties of the resulting cathode can also be improved by adding material that improves cohesion of the cathode material and adhesion of the material to particular substrates.

In a third aspect, the present invention provides an electrochemical cell comprising a positive electrode according to the description above, an electrolyte and a negative electrode (anode).

BRIEF DESCRIPTION OF THE FIGURES

In order that the present invention may be more readily understood, embodiments of the invention will now be described, by way of example, with reference to the accompanying Figures, in which:

FIG. 1 shows powder X-ray Diffraction patterns of the synthesised materials in accordance with Example 1 compared with the calculated patterns of the C12/m and R-3m symmetry lattice shown at the bottom and top of the figure respectively;

FIG. 2 shows ⁶Li MAS-NMR spectra collected on synthesised materials in accordance with Example 1. The spectra are presented in a stacked fashion with increasing nickel doping going from bottom up;

FIG. 3 shows first cycle galvanostatic load curves for the synthesised materials in accordance with Example 1; and

FIGS. 4A-4C show OEMS analysis of a set of the synthesised materials in accordance with Example 1.

DETAILED DESCRIPTION OF THE DISCLOSURE

The present invention will now be illustrated with reference to the following examples.

Example 1—Synthesis of the Nickel Substituted Lithium Rich Materials

The Formaldehyde-Resorcinol sol gel synthetic route was employed to synthesise materials with general formula

${\mathrm{Li}}_{\left(\frac{4}{3}-\frac{2x}{3}\right)}{\mathrm{Ni}}_x{\mathrm{Mn}}_{\left(\frac{2}{3}-\frac{x}{3}\right)}{O}_2$ with x=0, 0.06, 0.12, 0.2, 0.3 and 0.4 all the reagents ratios were calculated in order to obtain 0.01 mol of the final product.

Stoichiometric amounts of CH₃COOLi·2H₂O (98.0%, Sigma Aldrich®), (CH₃COO)₂Mn·4H₂O (>99.0%, Sigma Aldrich®) and (CH₃COO)₂Ni·4H₂O (99.0% Sigma Aldrich® were dissolved in 50 mL of water with 0.25 mmol of CH₃COOLi·2H₂O (99.0%, Sigma Aldrich®) corresponding to 5% moles of lithium with respect to the 0.01 moles of synthesized material. At the same time 0.1 mol of resorcinol (99.0%, Sigma Aldrich®) was dissolved in 0.15 mol of formaldehyde (36.5% w/w solution in water, Fluka®). Once all the reagents were completely dissolved in their respective solvents, the two solutions were mixed and the mixture was vigorously stirred for one hour. The resulting solution, containing 5% molar excess of lithium, was subsequently heated in an oil bath at 80° C. until the formation of a homogeneous white gel.

The gel was finally dried at 90° C. overnight and then heat treated at 500° C. for 15 hours and 800° C. for 20 hours.

Example 2—Structural Analysis and Characterisation of the Nickel Substituted Lithium Rich Materials

The materials according to Example 1 were examined with the two techniques: Powder X-Ray Diffraction (PXRD) which was carried out utilising a Rigaku SmartLab® equipped with a 9 kW Cu rotating anode; and MAS-NMR spectra were collected on the materials with a Bruker Avance III 400WD magnet.

FIG. 1 shows in the right panel: Powder X-ray Diffraction patterns of the synthesised materials compared with the calculated patterns of the C12/m and R-3m symmetry lattice shown at the bottom and top of the figure respectively. On the left-hand side the structures for the C12/m and R-3m are reported. It is possible to observe the presence of transition metal ordering around lithium in the transition metal layer for the structure with C12/m symmetry. The enlargement shows the details of the reflections associated with the cation ordering in the transition metal layer that is lost as the nickel content increases.

All of the patterns appear to show the major peaks consistent with a close-packed layered structure such as LiTMO₂ with a R-3m space group. Additional peaks are observed in the range 20-30 2Theta degrees which cannot be assigned to the R-3m space group highlighted in the expanded cut out of the figure. The order derives from the atomic radii and charge density differences between Li⁺ (0.59 Å), Ni⁺² (0.69 Å) and Mn⁴⁺ (0.83 Å) and appears the strongest in the structures of the low nickel doped oxides. In the Li₂MnO₃ end member the 2:1 ratio of Li:Mn allows for the formation of a perfect honeycomb and therefore has a near perfect fit to the C12/m spacegroup.

As Nickel is substituted into the material the long range in-layer ordering is progressively lost. This is mainly due to the substitution of 1 Mn⁺⁴ and 2 Li⁺ for 3 Ni⁺² to maintain the charge balance. Since Ni⁺² and Li⁺ have a similar atomic radius in comparison to Mn⁺⁴ the Ni⁺² will preferentially occupy lithium sites. Since the differential scattering cross section for X-rays of nickel and manganese are very close the ordering of the ions becomes less easy to resolve with X-rays and the peaks in the 20-30 degree range are lost. No presence of extra-peaks due to impurities was observed.

MAS-NMR spectra were collected for the NMR-active isotope ⁶Li on the whole series of materials. The normalized spectra in FIG. 2 show the presence of two signals, centered at 1550 and 780 ppm, corresponding to the two sites for lithium ions in the structure. The resonance centred at 1550 ppm is attributed to lithium belonging to [LiMn₆] and [LiMn₅Ni]/[LiMn₅Li] configurations, within the transition metal layer. On the other hand, the resonance at 780 ppm is due to lithium in the lithium layer.

A progressive loss of sharpness can be observed as the nickel content is increased although the chemical shift (i.e. the peaks positions) remains the same for all the materials. This result could be explained through two main phenomena: the generation of new local environments for lithium as nickel is progressively substituted to lithium and manganese in the lattice; and/or a higher concentration of stacking faults in the materials with high nickel content. In both cases the long-range order is broken leading to a progressive broadening of the NMR resonances.

Example 3—Electrochemical Analysis of the Nickel Substituted Lithium Rich Materials

The materials according to Example 1 were characterised electrochemically through galvanostatic cycling performed with a BioLogic VMP3 and a Maccor 4600 series potentiostats. All the samples were assembled into stainless steel coincells against metallic lithium and cycled between 2 and 4.8 V vs. Li⁺/Li for 100 cycles at a current rate of 50 mAg-1. The electrolyte employed was LP30 (a 1M solution of LiPF6 in 1:1 w/w ratio of EC:DMC).

FIG. 3 shows the potential curves during the charge and subsequent discharge of the first cycle for each material according to Example 1. All the samples present a high voltage plateau of different lengths centred on 4.5 V vs. Li⁺/Li⁰, whereas the presence of a sloped region at the beginning of the charge progressively increases in length with the amount of nickel doping. The extension of this region may be attributed to the oxidation of nickel from Ni⁺² toward Ni⁺⁴. appears to be in good agreement with the amount of lithium (i.e. charge) that would be extracted accounting for solely the nickel redox activity. Hence, as expected, Li₂MnO₃ does not show any pre-plateau region whilst the

${\mathrm{Li}}_{\left(\frac{4}{3}-\frac{2x}{3}\right)}{\mathrm{Ni}}_x{\mathrm{Mn}}_{\left(\frac{2}{3}-\frac{x}{3}\right)}{O}_2$ x=0.3 doped oxide presents more than 150 mAhg⁻¹.

During the first discharge, none of the materials show the presence of a reversible plateau, indicating a difference in the thermodynamic pathways followed during the extraction (charge) and insertion (discharge) of lithium ions from/in the lattice of each sample.

For all the material according to Example 1 the first cycle presents the lowest coulombic efficiency value due to the presence of the high potential plateau which is not reversible. The coulombic efficiencies appear to quickly improve from the first cycle values, around 60-70%, to values higher than 98% within the first five cycles. However, with this regard Li₂MnO₃ and

${\mathrm{Li}}_{\left(\frac{4}{3}-\frac{2x}{3}\right)}{\mathrm{Ni}}_x{\mathrm{Mn}}_{\left(\frac{2}{3}-\frac{x}{3}\right)}{O}_2$ with x=0.06 are an exception, showing an initial loss in efficiency. When the nickel substitution increases such that x=0.12 a significant improvement in the electrochemical performance is seen, indicating that there is a change in the nature of the charge storage mechanism.

Example 4—Gas Evolution During the First Cycle of the Nickel Substituted Lithium Rich Materials

One pellet of each material according to Example 1 was assembled into a Swagelok® test cell specifically machined to carry out an Operando Electrochemical Mass Spectrometry (OEMS) measurement. The mass spectrometry measurement involved in the OEMS experiment was performed with a Thermo-Fisher quadrupolar mass spectrometer. OEMS was performed on the set of materials in order to get an insight on the origin of the extra-capacity that is observed during the first cycle.

FIG. 4A-4C shows OEMS analysis of the nickel doped

${\mathrm{Li}}_{\left(\frac{4}{3}-\frac{2x}{3}\right)}{\mathrm{Ni}}_x{\mathrm{Mn}}_{\left(\frac{2}{3}-\frac{x}{3}\right)}{O}_2$ for x=0.2, 0.3 and 0.4, respectively. Each graph shows the galvanostatic curve during the first two cycles (top lines in each graph), the oxygen trace, and the carbon dioxide trace for each material. The right y-axis represents the electrode potential while the left y-axis the gas release rate expressed as moles of gas per minute per mole of active material, both axis reported as function of lithium equivalents. Argon was used as carrier gas with a flux rate of 0.7 mL/min and the electrode was cycled against metallic lithium at a rate of 15 mAg⁻¹ between 2 and 4.8 V vs. Li⁺/Li⁰ for all the materials. The electrolyte employed was a 1M solution of LiPF₆ in propylene carbonate.

CO₂ and O₂ were the only gaseous species detected for all the samples and a clear trend appears from FIG. 4A-4C, with a progressively lower amount of gas released as the amount of dopant nickel increases.

CO₂ is detected first in all cases, peaking at the beginning of the high potential plateau (around 4.5 V vs. Li⁺/Li⁰) region and progressively decreasing until the end of charge.

The amount of CO₂ decreases in line with the increase in nickel in content but is never eliminated. On the other hand, molecular oxygen appears to be released in a spike-like fashion that reaches its maximum towards the end of charge for the materials of the present invention. In the case of the high Ni substitution where x=0.4 it has been shown that there is almost complete suppression of O₂ and a strong reduction in the amount of detected CO₂ (FIG. 4C). This result is suggestive of the important role played by nickel as in stabilizing the oxide structures at high potentials by reducing the oxygen loss process.

Claims

1. An electrode comprising: a compound of the general formula:

2. The electrode of claim 1, wherein the compound is a cathode material having a layered structure.

3. The electrode of claim 2, wherein the layered structure is expressed as the general formula: aLi2MnO3·(1−a)LiNi0.5Mn0.5O2 wherein a is 0.2.

4. The electrode of claim 1, wherein the electroactive additive is selected from at least one of carbon or carbon black.

5. The electrode of claim 1, wherein the polymeric binder is selected from at least one of PVDF, PTFE, NaCMC or NaAlginate.

6. An electrochemical cell comprising a positive electrode of claim 1, an electrolyte, and a negative electrode.