Alkylspiroborate salts for use in electrochemical cells

A borate salt of formula I

• 5
• wherein
• M is a metal ion, a tetraalkylammonium ion,
• PRaRbRcRd, P(NRaRb)kRcmRd4-k-m
• C(NRaRb) (NRcRd) (NReRf), C(Rz)3, tropylium or
• a heterocyclic ring containing P, N, S or O, or a fused heterocyclic system containing a P, N, S or O-containing ring,
• k=1-4, m=0-3 and k+m≦4,
• Ra to Rf are H, C1-8-alkyl or C1-8-aryl(C1-C8), which may be partially substituted by F, Cl or Br,
• Rz is an aromatic or substituted aromatic ring,
• x and y is 1, 2, 3, 4, 5 or 6,
• R1 to R4 may be identical or different C1-8-alkoxy or C1-8-carboxyl radicals which are optionally bonded directly to one another via a single or double bond, and are optionally partially or fully substituted by electron-withdrawing groups which are
• F, Cl, N(Cn′F(2n′+1−x′)Hx′)2, O(Cn′F(2n′+1−x′)Hx), or SO2(Cn′F(2n′+1)Hx′)
• wherein
• n′ is 1, 2, 3, 4, 5 or 6, and
• X′ is 0 to 13.