  We study the competition between the Kondo effect and frustrating exchange
interactions in a Kondo-lattice model within a large-${\cal N}$ dynamical
mean-field theory. We find a T=0 phase transition between a heavy Fermi-liquid
and a spin-liquid for a critical value of the exchange $J_c = T_{K}^0$, the
single-impurity Kondo temperature. Close to the critical point, the Fermi
liquid coherence scale $T^\star$ is strongly reduced and the effective mass
strongly enhanced. The regime $T>T^\star$ is characterized by spin-liquid
magnetic correlations and non-Fermi-liquid properties. It is suggested that
magnetic frustration is a general mechanism which is essential to explain the
large effective mass of some metallic compounds such as LiV$_2$O$_4$.
